What they will tell you and what we have found to be true is follow your passion. Sell things you personally love. Sell things that you find interesting or you will quickly get bored with the whole thing.
I still crack up when I think about some of the weird things we have sold on ebay like a large pinecone that was found in the front yard, lol.
The one thing you need to remember is don't sell junk. Make sure whatever you sell is good quality merchandise.
Hope that helps :)
Tony and Deidre