The fact of the matter is that there are plenty of newer, faster, and cheaper alphas that run VMS and Tru64 for significantly cheaper than these 255s. Now, combine that with the fact almost all of them are 'sold' by one seller in private auctions, and it becomes pretty clear that it's some kind of scam.
Another fact of the matter is that some people who have a running system that works as needed/expected sometimes prefer to try and keep it running for as long as reasonably possible. One way to do this is to buy up systems for spares, so you can simply swap parts or switch disks when something goes wrong. In a old job I had, our department used to buy every HP 9000/300 that came on the market in Ireland or the UK, because we had to keep some ancient systems running software that was too costly to port or redevelop going 24/7. An hours downtime cost more than the price of 2-3 spare systems.
The XP900's aren't all being sold by the same seller, yet I've sometimes seen these 433/600MHz Alpha models go for 4 figures on eBay. I don't believe CPU speed is the reason some Alphas sell better than others.
It also happens with SGIs - Ian Mapleson has provided countless examples on here of companies who buy parts or entire systems at prices much higher than any hobbyist would believe that hardware to be worth. This fact alone isn't enough evidence to simply conclude that anyone buying/selling hardware at prices you consider to be high is involved in a scam.
BTW, its Nekonoko who has to deal with any legal threats that arise when you single out and make accusations about specific eBay sellers, so out of courtesy to him it might be wise to edit some of your posts.