I think buying a used rig from GC is just about like buying one from anyone else - - private party included. You do not know what you're getting, unless you know who it was who sold the item to CG, or who was selling it privately. Most of the transactions I've conducted at CG have been good - - the salespeople are sincere and have truly been interested in how I liked the product I was checking out; but I have always bought new from them. I have traded in some gear and everything I traded was in top working condition, but then again, I'm kinda anal and really try to take care of my stuff, so no doubt, someone probably got a good deal buying my stuff. So, that being said, there are good things out there. Here's what I'd do....
Take your favorite ax with you (you know and use that instrument, so it won't sound different to you than what you're used to) and if you have a friend who's into audio equipment repair, take him/her with you. Look for a rig that's physically clean-looking and hasn't been beat around the block. If you're checking out a tube rig, try to find out how new the tubes are. If the brand name and tube-type (numbers like 6L6 GC) have cooked or worn off, you know they're old. Then play the rig in every manner possible, loud, soft, with tone controls in all different positions, and with your ax plugged in (without playing), turn all the knobs slowly (including volume and master volume controls) all the way up. There should be no crackling, scratching, hissing (other than with the volume turned all the way up), feedback, hum, or the like. If it is a combo unit, the speaker should sound good; try another speaker with the head section just for grins and to be sure the unit's speaker isn't failing.
I hope all this info helps and doesn't seem in any way patronizing.....just trying to think of everything I'd do in your shoes. Best of luck!
Peavey Cirrus 5 String, MarkBass CMD121P (MBCM #329), MarkBass Micro Bass C108