I wouldn't have thought retailers would be taking part in Black Friday/Cyber Monday sales unless it made them a profit. Q4 is naturally busy for them with all of the holidays, but this weekend is def one of the busiest in the year for them - with lockdowns and the general publics feeling of unease about visiting busy shopping centres/districts then this year even more so.
I guess you can bucket the sales into something like people that will buy the product anyway and are just looking for a discount; people that are on the fence and the discount pushes them over the line and then people that didn't really start the day wanting the product and just saw a good 'deal'. I would have thought the second 2 categories more than make up for the buyers who would have bought anyway.
Plus, there's other costs associated to the seller that you can think about. These sales help to clear stock, so they have less warehouse space to pay for when next years lines are released and stock bought in. When that new years stock is bought in they'd probably see the old lines not selling, so would have to offer a discount anyway, and that discount would likely be more than the BF sale.