They also need to be able to replenish that stock at current prices. I’ve worked retail many times in my life and arguably kinda-sorta do so now (albeit largely over the Internet) and I’ve never run any store where we did not set our pricing by replacement cost rather than original invoice cost. In my current operation there are some rare exceptions for clearance items and the like, but for the vast majority of products we sell for what it’s going to cost me to get the next one to put back on that shelf, not what it cost me for the one I’m selling you now.

I don’t have any insider insight into other companies’ operations, but I imagine a lot of other retailers work things the same way. Especially these days.