Stamp Dealers buy large collections/accumulations of stamps.
They then sell off the good stuff to collectors and sell off the common stuff to us apostates who use stamps for postage. Overall they come out to the good on the whole deal.

I used to get about a 20% discount on mailing out eBay packages by wallpapering them with 2,3,4,5 cent stamps. Drove the window clerks nuts, which was a side benefit

I bought stamps this way for 20 years. Was still seeing some of the same sheets of stamps the whole time.

It's a real hassle to try to get the post office to take them back. Has to be a special window at a designated office.

If you bought a large chip collection, with say $200 in Hard Rock Chips, would you take them back to the Hard Rock for face value? If in Las Vegas, yes, otherwise you have to offer a discount to move them.