I've sold a lot of stuff that I couldn't otherwise trade. Check my signature to see my listings. I like it a lot. The selling fees can be a bit much IMO but it's easier to sell things than on Ebay as far as I'm concerned.
I sent one shipment to them and was very satisifed. The site can be great but remember a couple of things:
1. If items are slow movers on other websites you've tried they will also move slow at COMC no matter what their testimonials lead you to believe.
2. Check to see how many of your cards you plan on sending in are already there. Example-- there are many 2010 Ryan Braun cards listed however many of them have 20+ of each card in stock and go for less than .35. Also I checked 1989 Donruss Ken Griffey Jr RC total of 228!
3. If you are looking for cash be very careful that take somewhere near 20% of your sales in fees.
As a buyer I think COMC is a great site and prefer it hands down to ebay for singles.
I never thought about the selling fees there, but it does appear to be 20% if you want to cash out. I usually load my account with $20-$50, and then buy away. Anything I sell usually just goes back into stuff I'm buying.
This site is the best for buyers for sure. Basically, once you want your cards that you've bought, you pay $3 shipping for the first, 25 cents for all after that. Sure beats paying $2-$4 for shipping on one card on ebay...And you can get breaks on shipping if they run promotions or you have enough to use the bulk shipping option.