I can see that Cronosoft have listed two ZX81 cassettes on sellmyretro - https://www.sellmyretro.com/user/profile/cronosoft
Titles are:
Impact
Shapestack
Lazy Frog
Has anyone played these?
Cronosoft ZX81 Titles
Cronosoft ZX81 Titles
Rich Mellor
RWAP Software
RWAP Adventures
SellMyRetro
Retro-Printer Module
Also Involved in:
Icephorm
RWAP Software
RWAP Adventures
SellMyRetro
Retro-Printer Module
Also Involved in:
Icephorm
Re: Cronosoft ZX81 Titles
Not recently. I think I've looked at all of them in my time though. Impact is Bob's asteroids and the other 2 are 1K IIRC.