I've thought about this too. In theory, things sell for less in a small market than in a large market. E.g. that toaster that only brings $2 at a yard sale brings $6 on ebay because there is a larger audience of people looking for toasters there. We'll call this the markets spread
The trick would be to find things with a markets spread so big they justify the time and shipping costs.
How to execute on this theory? Probably 1,000 ways to do it.
One way would be to buy a junk car / motorcycle and sell the parts. People with a broke-down ride will pay extra for the convenience, and unlike the salvage yard, you have a national audience.
Another is to find antiques, art, or other rare objects for sale in small markets like yard sales or CL that you can resale on ebay.
A third way is to offer to sell all your friends' stuff for a 20-25% cut. Word will spread. There were some such shops in my town but they went under due to the costs of expensive prime real estate. A sole entrepreneur working from home could make it profitable if they could develop a referral network. Also, local advertising is cheap.