A lot of those sellers likely have stolen product, or product they get from freight auctions for 80-90% off MSRP. I get offered stolen LEGO a few times a month; usually 30-40 boxes at a time. I run a legitimate business so I don't buy these, but for people that aren't incorporated and just sell as a hobby it's pretty much a dream to be offered product like that for pennies on the dollar.
That being said, I do have a story about an auction a few years ago. I managed to buy a pallet of those Saturn V rockets a few months after they were released. There were about 40 of them in total and I paid $1,100 for all. Was a shipment to the local Amazon/Walmart (can't remember which) warehouse that got damaged and auctioned off. Happens a lot more often than you would think. In this case it's pretty unlikely that 100+ sellers all managed to find product from auctions, though. But each of them having 2-3 stolen boxes to sell is, in my opinion, quite reasonable.
Fortunately I've never had any trouble waiting out listings like that. As you said, it's less to worry about later.