THIS! I've actually monopolized (or very nearly) a few products by selling in reasonable stacks. I almost never have to change my prices because people are looking at net cost most of the time, not price-per-item. I think it's polite to keep things at a level where potential buyers can assume what their mats are going to cost. If someone tries to undercut me, I take all my stuff down, let the supply dry up and check back in a couple of days. It pays to make things easy on the buyer.