[Hectic]

For used stuff it's best to buy local. That way you can go take a look before you commit. Trader ratings are helpful however used stuff is used. Just like a vehicle you never know how it was maintained. Who did the work on it. What kind of parts they used etc... so you must be aware that you may get a good solid gun or you may wind up with a bunch of repairs the first time you fire it.
I will give this advice, without trying to sound leetist or something.
If i was gonna buy a low end gun (cyma, jg, the entry level gandg stuff like you are looking at) I would buy new because let's face it they aren't that expensive, and they aren't that quality so at least buying new you will know how many rounds and what abuse it suffers.
If i was looking higher end (vfc, LCT systema) I would browse the classifieds for a deal but likely would just buy new unless I could save 40% or more or get a bunch of "freebies" like mags and sights n such