For a store it really is that simple, and they do it all the time with really good sales (esp. loss-leaders).

The "preferable" regime is whichever one is easy to implement, appears fair, and ensures primarily that the store has a reputation for things being in-stock even when on sale.

Which is usually max 1, 2 or 4 per customer. If you really want more, you can usually put them in your car, go back in, get in line again and pay again. If you do this more than two or three times, a manager will probably ask you to stop and prevent any further. But you can always come back tomorrow.

It works pretty well. People who really need double or triple can get it with just a little extra work, but it absolutely prevents people from clearing out the store's stock in a single, quick easy transaction.