There are 2 types of items and related business models BG have with their deals:
95% of the time it's ipads/laptops/kindles etc. These are merchandise that they resell at profit elsewhere but arent really valuable or limited, just that they are available at a sale price and in volume they can be resold with a profit. Most buyers have no interest would ever buy an ipad to resell somewhere for 15% more.
Then there are the collectibles. These can be worth vastly more than the price paid and the commission offered. We all know about the 2019 silver eagle which went from under 100 to 2k. /sneakers/tickets can fall in this category too. In some cases they are worth many multiples at the time of purchase, many times this unbeknownst to the buyer. This is where it can be viewed that the BG are taking advantage of the buyer. True, in a few cases perhaps the buyer has zero interest to resell, but the vast majority would prefer to keep if they knew the real story.
And both are the same for the buyers. They don't when the ipads are on sale without the BG telling them. They don't have time/knowledge/resources to know if the ipads are profitable. They just know that that there is a guaranteed buyer on their end, and they are happy with that.