I don't actually know the answer to your question. I'd be surprised if ebay had anything where you could restrict buying.

But one thing that popped into my mind was maybe you need to charge a little more for your items.

If people are jumping in immediately to buy all 20, that says they may be planning to on-sell them at a reasonable profit & are confident they can do that. So charge more-maybe even double. See how they move then as it may knock the entrepreneurs out of the picture a bit & leave the collectors.