I forgot to add one thing, which IMO is the most important factor. In order to be successful you can't collect AND make a living. I would want to keep too many items for the PC which would mean I don't sell and couldn't make a living. In order to make a go at this you'd have to be willing to sell everything and keep nothing. Personally, I'm too much of a collector to make a living selling.
I asked my LCS owner this once and he said it's not possible (at least for him). He has to be 100% committed to selling everything and not collecting himself. It's like how you can't be a "little bit of an alcoholic". The temptation to keep and collect cards would hurt profit too much.
Current needs: Kirby Puckett Relics, Game Used, Patches, Inserts, & Parallels. Also looking for BGS 9.5 Topps RCs from 1981-current.