Alot of people here are ragging on the Classifieds. I've bought three aeg's, two grenade launchers and a gbb off the classifieds: two stock TM's (mp5sd5 and mp5k), my upgraded CA m15a4, a CA m203, a G&P gp-35 and a KJW m9. I am perfectly happy with all of them, great deals. The only thing I would advise buying new is things that hold gas (mags, grenades), they're finicky things that need to be babied and there's no guarantee used ones have been. That said, I have bought the bulk of my gbb pistol mags used and have had no issues. You just need to do your homework. Research the item, research the seller, and ask for specific details on the item's history and condition.
What a previous poster said about cheaper items is true, the higher quality the item the more-likely it has held up to previous use. Also the higher the quality the more expensive the initial price tag and thus the more likely the previous owner took pains to take care of it. I've sunk over $1000 into my baby and I'd rather get shot then take a dive that would potentially damage her.
Another thing to remember is if the price tag looks too good to be true, it probably is. An ad for a TM or such for $100 is either a scam or one seriously beat-up weapon. That said there are some really good deals in the classifieds. Sometimes people need cash quick, especially around xmas time. Knowing which is which comes back to homework.