Is it possible?

I have a few buyers who buy up all of my items as soon as I list them (like, I'll list quantity of 20 of something and they'll buy all 20). I want to stop this happening if I can.

I know a sale's a sale, but many of my buyers are repeat, and these items are collectible, so I'd prefer to restrict the quantity (eg: maximum 2 per customer) to allow multiple customers the chance to buy these items.

Thanks in advance.