Sorry, I probably should've actually given some input
I will say that I've been on COMC for a couple months now and as a seller, I am EXTREMELY pleased. There is a bit of lowballing, but you just need to learn to cope with it. If your card is in enough demand, it will sell eventually.
I've sold a handful of cards at above "book value". I've done a few quick flips for nice money. From a prospecting standpoint, I've recently been buying up autographs of guys that I think will appreciate down the road and just keep them 'Not for Sale' in my inventory until the time is right. You'll learn how things tend to go as you spend more time there.
My only frustration of late has been occasional site downtime. It hasn't been an issue for me VERY recently, but there quite were a few outages (some a half hour or longer) in the past month. Hopefully they're working to resolve it and uptime can be kept as close to 100% as possible.