Ebay and COMC is a way to supplement my hobby addiction. I think it would be incredibly difficult to make a living at this. I basically use ebay/COMC to sell what I don't want in order to buy more of what I think I need and want. I've sold $800 on COMC which I know is not a lot. But it's $800 less that I had to pull from my own pocket. Win win. Same on ebay. I don't have huge sales, but what I do sell I use to buy more PC items and singles. There is nothing better than having a few hundred in the paypal account from junk sales. Basically free money!
I think it's possible to make money at ebay over a short period of time (hit a nice string of lucky wax breaks, hit a few prospects timely, grade and flip a few high end cards at the right time) but I think over the long run with the amount of time you'd have to spend and the fees involved making a 'living' at ebay would be incredibly difficult. At least in the sports card arena it would be, not sure about other avenues or items for sale on ebay.
Current needs: Kirby Puckett Relics, Game Used, Patches, Inserts, & Parallels. Also looking for BGS 9.5 Topps RCs from 1981-current.