Hi Katie and welcome to the forum,
Most people do it the other way, use dropshipping to test market products and then if they find a strong product, then they will stop dropshipping it and move to a wholesale model and buy in stock. Of course the advantage of buying in wholesale is a cheaper price than dropshipping offers you, so you're making more margin that way.
A big problem you might find trying to test market in this way is that you may well get stuck with stock that isn't selling at all well, so then you have your money tied up in inventory that you can't move or that you may well lose money needing to move cheaply to regain cashflow to move froward. So it could have the exact opposite effect as to what you are trying to do.
I wouldn't recommend doing the way you are suggesting personally. Yes, you won't have your money tied up in inventory, but you won't have the same profit margin per sale either and loosing margin on a platform like eBay is crtitical to success. You really need as much margin as you can get to insulate yourself from competition when or if ever needed.