It's a question we see over and over again here on the forum.
Of course if budget isn't a concern, then the easy answer is to just pick one or the whole 10 of the consistant top ten best selling performers. Then just ring your wholesaler, nego a great per item price based on a container load purchase, then unload them on eBay at a reduced margin to create market dominance and make margin through a fast sell through rate that will give you a high turnover number.
That's all pretty simple, but not so simple if you don't have the budget that will allow you to use that sales strategy!
If you're looking more closely at dropshippping as your preferred supply method, then there isn't much point in trying to sell any high sell through rate on eBay because the chances are that you will be going up against sellers using the exact or very similar scenario as I've outlined above, and you're going to go round and round in circles and still be out gunned pretty easily!
So unless you have an unlimited budget, then it's not about "what the best thing to sell on eBay is" it's more about "what's the best thing I CAN sell on eBay" and the only way you can possibly find that out is through research, research and then when you think you've done enough research, do even more research!
While dropshipping offers sellers a very cheap way into the market, it doesn't offer an easy way into the market.