How's this. I want to buy a new graphics card. It retails for $200. I offer to buy it for $150, the store makes a smaller margin but still profits.
You might not wait a long time to get a pizza but would be willing to wait a few days to see if you can get the new cellphone or laptop at a cheaper price?
Still suffers from the problem that in order to obtain the discount, a bunch of other people would have to request the discount on the graphics card in those few days. That's unlikely so I probably wouldn't bother asking for the discount (in turn, everyone else is thinking the same, which makes it even less likely).
You might not wait a long time to get a pizza but would be willing to wait a few days to see if you can get the new cellphone or laptop at a cheaper price?