I remember your post regarding the orange spot of feathers he has coming-in on the side of his face/head, this was just a few weeks ago at most. I'm the one who commented to you that these orange spots on each side of their head is a normal marking that comes-in on all Yellow-Sided Green Cheeks, and told you that my Yellow-Sided, Bowie, had his grow-in when he was around a year or so old. He's now just over 2 years old.
So that would make your guy what, approaching a year-old or so now? You have to take a step-back for a minute, put your anger and more your frustration aside, and realize that your bird is going through the most difficult time in his life right now, and it's probably going to last for another 6 months or so, until he's at least a year and a half old, if not 2 years old. I speak from experience, as I just went through this with Bowie, I got him when he was 12 weeks old and like I said, he just turned 2 years old in June. And the good news that I can give you is Bowie has been back to being a total little sweetheart since he was about a year and a half or so old, just like he was when I brought him home at 12 weeks. So you need to remember that this is completely temporary and will soon pass.
That being said, you cannot allow your frustration, disappointment, anger, hurt that he's biting you, etc. get the better of you throughout this process, because I can guarantee you that if you just keep treating him the same way that you always have while he's going through all of this stuff, then he'll be fine when he reaches the other side of it. However, if you do in-fact let your emotions and frustration get the better of you, and you do things like "hit him", yell at him, stop paying the same amount of attention to him or stop letting him out of his cage as much, start ignoring him, etc., then when he does get through all of this stuff he's not going to trust you any longer, rightfully so, and then you're in trouble. They are extremely intelligent and they remember everything, and they hold a grudge. Once you lose their trust and their faith you can get it back, but it's really difficult and they make you work for it. You can't simply like your bird when he's being all nice and cuddly and snuggly and pay attention to him when he's like this, and then hate your bird, yell at him, hit him, or ignore him and spend less time with him when he's hormonal and going through the largest molt of his life. This is the equivalent to a human parent treating their 12-14 year-old teenager who's going through puberty this way; yes, they're horrible people during that time period, lol, but it's normal, it's temporary, it's a natural part of life, and if you truly love them then you love them no matter what.
Your bird is going through puberty, his first hormonal rushes/rages, and he's also at the same time going through the largest molt that he ever will go through, shedding his baby/juvenile feathers and getting his adult feathers, which for a Yellow-Sided Green Cheek are pretty extreme, you're going to see that the coloring on pretty much every part of his body is going to change drastically-on his face/head, on his chest and belly, and on his wing-tips. So he's moody, irritable, and itchy as all-hell. He's miserable, he's cranky, and yeah, he's going to get nippy for no reason other than he's miserable. And you either help him through this and are there for him throughout it, no matter what, or you need to find him another home with a loving, experienced bird person who will love him no matter what...But I'd hate to see you do that, because he no doubt loves you very much, is already bonded to you deeply, and re-homing him during this time would just be devastating for him.
I'm hoping that you saying that you're thinking of "selling him" if he doesnt' stop biting soon is just you venting to us and is not actually something that you're seriously thinking about doing. You need to realize that this is not anything that is specific to your bird, this would happen with any bird that you bring home as a baby that hasn't yet gone through puberty or their first, big molt. So it's not like he's doing anything different or specific to him, and you can get rid of him, go out and buy another hand-raised baby bird, and they aren't going to go through the exact same thing...they will.
You definitely cannot "hit him" or yell at him when he bites you, that's not right under any circumstances. What you do need to do is to let him know that biting is not acceptable, and with Green Cheeks, who are "velcro-birds" and who absolutely love and crave attention, the best way to show them that biting is not going to be tolerated is by using the "Shunning Technique". If you use this technique every single time he bites you, he'll quickly get the picture and he'll stop, regardless of his hormones. And you won't lose his trust by using this technique. It's pretty simple, all you do is any time he's on you/with you and he bites you, you say a phrase like "NO BITES!" very firmly, but not yelling at him, just firmly say it, and then you immediately put him right down on the ground or on his cage or stand (I like the floor because they hate being put down on the floor, works better in my opinion than putting them on their cage or a stand). As soon as you put him down on the floor you immediately turn your back to him, and then you have to totally ignore him completely for 5 minutes. No more, no less. Don't talk to him, don't even make eye-contact with him, ignore his cries or screaming, etc. Walk into a different room if he starts to climb up your leg. Just pretend like he isn't there for 5 minutes. And then after 5 minutes you go and get him to step-up again, and if he bites you again right after you pick him up, then you again say "NO BITES!" firmly, put him back on the floor, turn your back, and ignore him again for 5 minutes. And just keep doing this every time he bites you, without fail. He'll get it very quickly. Usually what happens is that they get so upset and are so wanting your attention that they curb their biting pretty immediately. And the other side of this is whenever he allows you to pet him, touch him, whenever he steps-up, etc. without biting, then you need to really verbally praise him a lot and give him his favorite treat. Keep treats in your pocket or around the house where you interact with him so you'll always be able to reward him for being a good boy. Again, they love your attention, they love treats and verbal praise, and they absolutely hate being ignored and put down on the floor.
He'll get through this, it won't last very long. Like I said, I just went through this (Bowie actually latched-onto my hand and my foot and wouldn't let go, I actually had him hanging off of my hand in mid-air, and it was nothing but his hormones going absolutely nuts)...Something else to remember is that right now he cannot be allowed to have any "nesting" like places that he can go to or get into, and no nesting material...So if he has access to any type of tent/boxes/hammocks/huts etc., any small, dark, warm places inside of his cage or in your house (underneath furniture is a big one), they all need to be removed and blocked-off, as this will put them into a horrible hormonal rage, and I emphasize "rage" pretty immediately. That's what happened to Bowie when he latched-onto me those two times and wouldn't let go, he had wondered under the couch and was under there quietly talking to himself non-stop, and when I tried to get him out (I had a flashlight) he totally ignored me, he was looking right at me but it was like I wasn't there, he just stared at me with a really odd look in his eyes, kept mumbling to himself, was all fluffed-up, and when I started to stick my hand under the couch to get him out, he literally "charged" at me, running full-force and latched onto my hand as hard as he could. And when I pried him off he ran right back under the couch again, and again he came running full-speed at me and then latched onto the skin on the top of my foot (my god that hurt). I had to pry his beak apart to get him to let go, and the minute I got hold of him and he knew he wasn't going to be able to get free again, he immediately got all cuddly and loving again, and was wanting to give me kisses and calling me "My Baby" again...yeah, right. But it really was like he suddenly snapped-out of a kind of trance that he was in due to his hormones being activated and rushing due to him being under the couch in a small, dark place.
