is there a history with seeing a vet? adopting from a private seller, you should take him/her to the vet anyways to make sure there are no issues and to get a good baseline for future visits, I would address the issue with the doctor to rule out disease or sickness. If you have other birds then the new addition needs to be quarantined till sickness is ruled out.
hopefully this is not the case, but the seller might be trying to pawn a problem onto someone else...
taking a quick look it seems to be that this guy plucks mainly on his left side. Could be an indication of skin irritation or pain of some sort, maybe a minor infection localized to that area? The first picture shows it the best, you can almost see his skin between the messy feathers. The third picture you can sort of see it too but not as well due to the lighting.
Of course it could just be due to stress or a learned behavior. It strikes me though that maybe the owner has noticed their parrot plucking and either wants rid of them because they're not perfect anymore or they don't want to sort the vet bills. There's stupid reasons people get rid of parrots