It isn't only whether new stock will be available, it is the wholesale price they expect to have to pay in order to restock.
Beyond that, there's the question of when
and how much
restock they get.
Say a retailer has a given item, say a $1000 rifle, that they usually move ten of in a week. Now they're getting five a week. It wreaks havoc with cash flow, and the dealer has to make more off what inventory he can get.
If demand has spiked and supply has diminished, that's going to cause a rise in prices. It's Econ 101, not Introductory Ethics.
There's a line, of course, but defining it is difficult. Joe Bob selling something at a 300% markup to make a quick buck might be infuriating, and he may be a jerk, but is it really outside the bounds of what we expect in a free market?