Those are the Zebra Systems ones. When I first saw them come up he was flogging them at $49.95 USD (1999) each. When he started getting orders the price jumped up ($99 USD in 2000) every year ($200 USD in 2008) or so.
Perhaps he's finally running out of stock :o) I was more interested in the support peripherals (memory packs), but I think he's priced himself out of the market. Website say's he's on holiday, but it's been a long holiday if the groups are anything to go by. Most of the links are gone in support. So I'm sure he's done his dash and run.
I wouldn't say "all the time."
Cor, I must have been drunk with joy and over enthused with the discovery. If we focus on one make, then it is very infrequently, but across all systems, a couple of great finds a year seem to get mention. It's getting harder to pick the true 'barn finds' vs the recreation projects, but I stand by my comment, there are still new old parts comming to light every so often. The useful things like cases or custom chips/programmed ULAs are almost non existant, but unpopulated circuit boards and chip packs are all over the place. It's the oportunists that get lucky like mr. ZX81 there, make a splash and scam, it hiding the good stuff out there.
I'll go crawl back under my rock again :o)