Many factors are included here. Disposable income has crashed as necessities are way up in price. This leads to buyers either scoffing at retired sets, or finding alternative means of acquiring similar non-branded versions of the sets. Sellers have to chase prices if they want to make sales and move product, leading to even further undercutting.
Lego is also yanking a ton of previous aftermarket cash by pumping out so many excellent sets now. If they keep it up, it will continue to drain the pockets of the consumer as they will have to battle buying a discounted set at Walmart or paying a premium for an aftermarket retired set.
It's simply a buyers market right now as those who can afford the upcharge on aftermarket bricks don't have nearly the competition that existed a couple years ago. The long term problem is that the increased price of necessities has normalized, and they're not coming back down. So this squeeze on the consumer is going to continue, which is just going to squeeze the reseller.