those previous replies are great, and a good general way to view equipment for sale, but its missing the details. its always in the details. this is mainly about the guns/markers, but its good for everything.
1) ask about the screws. many people will off load a gun because they used retard strength to put on or take off screws, resulting in the heads being stripped. in some cases it is an easy fix, but that's not what you want when you buy a new gun. these are common on the frame, grip and eye covers.
2) if the pictures are suspect, ask for a new set and make sure that you get both sides and the possible wear points, like where you put your hands. if someone is giving a hard time about the pics, then move along. it is either a scam, or its in worse condition of than the pics show. i like to ask for a "Proof of Life" pic. its a picture of the item with a USA Today newspaper(though you can use anything SPECIFIC). that way you can control what's in the picture, and can assure that you are getting up to date pics.
2.1) also, ask for both sides. i personally hate when someone posts gun pics, but only shows one side, or even multiple shots of one side. either there is rash on the hidden side or its suspect.
3) look for anything odd. if there are mismatched screws, or if the pic isn't as described. its your money, wait for new pics.
4) look for anything out of the ordinary. gobs of teflon tape on fittings. mismatched parts, misaligned feednecks, or feednecks that aren't sitting the way they are supposed to. there are plenty of pics of guns on the net that show the gun in stock form.
5) don't be afraid to ask questions. i'm honest, so i'll tell you how long i've had it, any damage, what the condition is. i ignore posts when people put numbers for condition. who is rating the guns and where is the providence? is anyone putting their rep to stand by their rating? pictures are a 1000 words...
6) understand how you baby something, isn't how other people baby it. which goes with the above. understand that you are getting something that may have problems.
most importantly, never be afraid to walk away at a deal. not to say that you can't pay the asking price, but more so if anything seems off, then walk away. if you are worried about it, then its not worth it.