You can research this yourself by putting an item into the ebay search and tick on completed listings. That way you can see what sells and for how much.
look for misspellings and resell them spelt correctly.
but then thats standard practice for ebay.
otherwise you need to find a good suplier and start a shop.
The only way to make good money on eBay is to sell products you either know a lot about - such as the collectors' market or art market- or products you have a wholesale contact with.
Don't try dvds, cds, memory cards and so on - the competition is far too great.