If you become invisible, and stay away from the shop, you can mostly avoid this effect. Not a permanent solution, but a temporary one at least. If you're going for the mystic, say, you can enter DT invisible and you should be able to teleport over without being bothered. The shoppie will eventually notice you while invis though, so don't spend too much time near the shop.

Shop restocks are based on the number of items currently in the shop. Roughly, the shop's capacity is the number of floor spaces + number of wall spaces, and will only restock if there are fewer items than this. So if you've been selling a lot of stuff in the shop, it will never restock. If you wait around for awhile when the shop is first generated, you can usually get a restock of about half the shop.