Don't know if you've looked but the one selling a disc for £566.91 seems to have undercut the one selling the same disc for... £566.92.

Both used.

The high ones are private dealers hoping to make a few bucks on a single copy of a title that is theoretically rare, Mort. It may have been deleted in some territories or never have been released, for starters. Some folk will pay a huge amount for a copy of a deleted recording that they need to complete a set, though quite often by searching around the same item can be found for much less elsewhere: that Gerhard Fourth Symphony that I posted has gone, even though the rest of the cycle are still available: it's a real hen's teeth record, and had a certain very, very, nice young man not located one for me in South London last year, I'd be looking at eBay regularly and seriously considering that copy even at that price. It's quite obvious that some dealers really don't have much idea of a disc's value, and fortunately they can be known to under-price discs as well as ask too much for them.

The cheaper ones come from the pile'em high and sell'em cheap brigade, who realise that world-wide trade in discs can be quite busy, and buy enough copies to negotiate a bulk price from the supplier.