Results vary, you could have a guy with high feedback and still sell you a piece of shit.
Usually though, if their feedback is good, they won't, but just putting it out there.
Some guns you buy used depend on the owner, each owner are different. Some people are OCD and clean them and maintain them 120%, where as some are "whatever," "in the closet" etc...
SPECIFICS for AEG's, I can't tell you since I'm a GBB kinda d00d.