I doubt anyone will tell you what they sell on ebay, since more competition will drive prices down. I have a few things I regularly pick up at garage sales that do well.
A lot of people I know (me included) make decent money selling books. Not just any old book, but some pretty specific valuable ones. There are a few authors I like and I know which books are collectible and rare, and I run into them for 25c at a sale and ebay for $20-30-40. Not worth messing around with if it's not that much profit.
If you're into science fiction, there are a lot of science fiction books you can find at sales that are rare. It's a lot of work to figure out which ones they are, but once you know what they are, it's easy to spot them.
I know a few people who make $$ online with porn sites.