There should always be a market on eBay for smaller sellers of used items. The bigger sellers are mostly new or I guess refurbished. After you get some ideas of what sells on eBay (You can look at the completed listings on eBay to see if a particular type of item sells well there), you would need to get some stuff to sell. You could try your local thrift stores, Goodwill, auctions, yard sales, Craigslist, etc.
Or, rather than selling a physical product, maybe try offering a service on eBay. Maybe writing articles, creating graphics, whatever it is that you would be good enough at to charge for that you've learned from your eBay selling experience or whatever.
Seems like you would be past this stage and of this stuff already (not sure how sustainable it could be), but you could try "eBay arbitrage", where you buy something off of eBay and then resell it there for a profit. Some sellers do not include important keywords in the title, give a poor description, poor pictures (or none at all), etc. All this stuff can scare potential buyers away, decreasing the competition and probably the final price. You could possibly buy the items and make a much better listing, and possibly profit.
Sellers may misspell the main keywords in the title as well, meaning most potential buyers will never find them. You could try a misspelling search using a site like Typojoe.com to hopefully find some of these items and possibly get some great deals.
You could also use a site like Ebuyersedge.com to set up saved eBay searches. You'd get an e-mail whenever a matching item is newly listed. Especially good for "Buy It Now"s that are priced right. You could set up your saved searches and then from there all you would be doing would be checking e-mails when you get them sent to you and looking at the items, so there should not be a whole lot of time involved in setting up your "system".
Or, if you find some listed an auction format, use a sniping service such as Hidbid.com to place your bid for you. It'll bid in the last few seconds, helping you to save money and avoid shill bidding.