I've been using them for the past 6 months. Great service. I've only had a problem once, and they corrected it. The first game I bought on the site (lol), never came. I received an email from Glyde, informing me about a month later that the game was shipped to their warehouse and that they shipped the game back out to me. I received it, and that was that. I've bought and sold around 15 or so games on the site. Never had a problem with it besides that first one.
If you're looking for something to make cash fairly fast, I would pass though. It takes about 3-5 days to receive the envelope to mail the game, then you'll end up waiting another week for the other person to get it. That and however long it took you to sell the game. They also have around a 2 day period to respond on the site indicating if there was a problem with the game. If they don't do anything, Glyde assumes that there's no problems and releases the money to you. You can then directly deposit it into your checking account or have them mail you a check (or use the money from your account and buy something on the site).
I recommend them to anyone who doesn't have a problem waiting for a week or two to buy and sell a single item.