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.
Always looking for Kirby Puckett relics, game used, patches, inserts, refractors & parallels. Also looking for BGS 9.5 RCs from 1981-current.