Hello Billy, not sure where you might be looking for used cages at, but your assumption as to why a used cage might be available is probably right, as would be that the bird may have died, though the seller may be reluctant to advise you as to the cause of death.....
Something you'll want to consider about used cages is that because of the way many are constructed, your good deal may well bring home an invasion of bugs.....when buying used cages, it is usually wise to disassemble the cage and carefully treat/disinfect/sterilize it before letting your new bird in it or even before bringing it into your home.....another consideration is that if a previous bird died in the cage, in some cases, the pathogen that killed the bird could very well continue residing in and/or around the cage for 60 days, 90 days or even longer, especially if the proper disinfectant/sterilizing agent was not used, so without knowing what you are disinfecting/sterilizing against, you may well be wasting your time...or need to disinfect.sterilize with two or more agents...but be very careful with mixing chemicals/agents.....
You asked about coatings.....many cages today are powder coated...a type of paint that is applied dry, using electricity to activate it and actually melt it to the cage metal, a much more durable finish than using using liquid or solvent based paints.....
Keep in mind that when washing cages with soap & water, the various tubings need to be thoroughly dried before putting the cage back together, to help prevent rusting because finish coatings are applied to the outside of the metal, not the insides of the tubings.....
Here are several links for cleaning, disinfecting & sanitizing:
Cockatiels, How to clean and disinfect a cockatiel and pet bird's cage birdcage.
http://www.peteducation.com/.cfm?c=15+1794&aid=2837
How to Disinfect Your Dead Bird's Cage | eHow
https://www.dropbox.com/s/ff1rzesec0bxzzw/Cleaners & Disinfectants.doc?dl=0
Good luck.....