the world of sports trading cards is completely out of control. if you are unfamiliar... the valuable "autographed" cards are no longer obtained by fans directly from the athletes. "hey sign my card !!". Nope, those kinds of autographs are worthless.
the autographs are now obtained by the manufacturers like "Topps" and "Upper Deck" on designated labels which are then certified and applied to the tradings cards and packaged randomly in sealed packs. Sealed packs can run anywhere from $1 all the way to $100+ per pack, but the buyer is never guaranteed an "auto" inside (by the manufacturers anyway). Sealed "hobby boxes" containing multiple packs often guarantee 1 "auto" inside one of the packs, but you don't know WHO you'll get. These boxes can cost anywhere from $40 to 1k depending on the product. People buy this stuff, get a crappy auto and then come right back for more looking for a big score.
Buying these boxes or sealed packs and hoping for a nice auto inside is somewhat to akin to gambling. The buyers will straight out tell you they are addicted and they often over spend their budgets looking for a good auto.
What to sell on ebay ?? Well... selling unopened and sealed cases/boxes of cards seems like a nice propostion. The market is way way up. I'm not sure how you get licensed to buy the stuff directly from the manufacturers at bottom dollar, but that would be the way to go if possible. If you can't buy from them, then you have to buy from a place that is when the product is first released and then hold onto it and let the value go up. With certain products, the box values do nothing but go up as a season progresses. So even if you're buying from a middle man licenesed shop, you can still make nice money by just sitting on the sealed case or box.
Of course their are scammers too ... there are wholesalers or shops that use micro instrument scales, open a sealed box and weigh each pack looking for the one which weighs a tiny bit more which is of course the autograph label. These packs can either be opened by the shop getting an instant autograph or they can sell it on ebay literally guaranteeing an an unknown auto. You'll see some addicted buyers drive the auctions as they search for a holy grail auto, like adrian peterson for example. These scammers can also misrepresent and sell those packs that are known NOT to have an auto (after weighing) as a pack that MIGHT have an auto to unsuspecting buyers. Or they can display an opened box of high quality material in their actual brick and mortar store where people walk in hoping to get an auto (which was already taken out). The sellers can even reseal the boxes after swiping the auto inside and replace it with a dud which is really sick.
Again, I'd recommend buying sealed cases of a good product upon it's release and then wait for it's value to jump. It's all pretty much wide open.