For Ebay, research is key. The pulse.ebay page is good for finding some top selling categories, but won't give you the kind of detailed info your gonna need. Price is key for selling on ebay, and you have to go thru listings and see what things are going for, and if there are bids, etc. The usual top categories of electronics and such aren't even worth attempting, since the competition is so high, and you won't be able to compete with the prices. Best to go after low hanging fruit. I've had luck with the following: collectibles such as dishes, nick nacks, tableware, . Women's clothing and shoes do well, especially designer brands. Kids clothes are hit or miss, you have to do some research on latest styles. Fragrances new or slightly used will sell every time. Old toys that are in mint or near mint condition, ones that are in the original package will bank some real $$$. Seasonal decorations, believe it or not, are hot items. Especially Halloween and Christmas. Coins, depending on the market. I made a small killing last year on franklin and kennedy halves, washington quarters when silver spiked to $40 an ounce.
Just some ideas for ya. As far as acquiring the stuff to sell, that's a whole other ballgame. I have a few good thrift stores and Goodwills nearby that I visit a few times a week. You'd be surprised at the stuff you find there for cheap. Pawn shops aren't bad either, hit or miss. Storage auctions are good, but the competition is stiff with the tv shows out now. Garage sales and estate sales used to be my staple, but people are getting smart now and doing research before they sell. You can still find some cheap stuff though, it's not as easy as it used to be. Craigslist can yield a diamond in the rough now and then too.
Good Luck!