What to sell on eBay or any auction - it's a widely-known secret...
That secret is: stuff that sells!
Let me explain how this can make you a ton of money:
eBay is a niche market. A big one, but still it's a niche. Probably a niche of niches. But stuff that will sell on other venues won't necessarily move on eBay. I've cross-checked several research tools and find that not only sales, but also keywords are different for eBay titles than in the "real world" of SEO. Hot niches you find on WordTracker don't necessarily sell as well on eBay.
So: again, the butt-ugly advice -
"Don't waste your time with trying to pitch things that a lot of people don't want."
Now, just because they don't want to buy your things at an auction doesn't mean they won't want to buy it online or in person. Selling fresh beef is best done over a counter, or in person from the farmer who raised it. Trading cards might sell much better from a booth at an industry event - or a convention.
There is this fascinating trend picking up - Amazon is continuing to expand while eBay shrinks. What this is attributed to right now is that buying habits have shifted over to wanting fixed prices. This last area is one where eBay is still expanding - and have changed their policies to adapt.
This means that Froogle and other such sites will continue to gain as well.
So put your best foot forward and write great sales pages for you items - and continue to promote them via social media while you only put those items which sell well on an auction platform over there. Keep the rest on your website, where the true niche denizens will migrate over and buy.