Selling some of your stuff to clean house and starting a side business are two very different things! I've done some of both.
Sold a few dozen records a few years ago -lps - and learned that jazz and pop from the 50s-on in excellent condition are worth the effort. Especially jazz. Sold all of it to Germany and Japan.
Sold some very good old clothes - and at least managed maybe 25% on a few things, which is nice when it's cashmere/Armani etc.
Later happened into a chance to buy some very high-end sunglasses and did some real business for a while, a good sideline that netted me several thousand bucks.
You can sell almost anything on eBay - have you used eBay yourself as a buyer? It is good to understand the way things really work. You don't need to pay for some eBay seminar if you take some time to figure it out.
One thing I will recommend if you decide to sell: if you have anything that would be appealing to an international customer, consider selling to Europe, Japan, Australia, South America ... with the dollar so low, I was selling Italian sunglasses to Italians, and they were still getting a great price, even with $22 priority international shipping.
If you just want to know what to sell, or anything about pricing, etc, just use eBay for your research. It's easy enough to search items and see what's up.
The emphasis shifts from year to year - at the start it was mostly collectibles, then quickly included anything you would see at a good garage sale. Now the majority of the stuff is sold by full-time sellers. I buy my printer ink on eBay, books, cds, electronics ... it can be great. But treat your buyers honorably!