I would say for your first AEG/GBB you want to buy new from someone who can remedy things if it arrives DOA.
I've seen a ton of new players vanish after their rifle eats a piston or just stops working. They've got no extra cash, and no experience on how to fix it so they're out. Sometimes for a month, or the season, or even for good.
Once you've been around the block there is a ton of good stuff in the classifieds. Folks getting out with rare AEGs, guys who compulsively buy the latest and greatest and then flip them when something newer shows and more. You just need the street smarts to smell out the good from the bad.
Airsoft, where nothing is hurt but feelings.