Printed one today, this is so fun. like 90% of the fidgety fun of an OTF for not even $1 in plastic. He's even got variants you put utility blades in so it can actually cut stuff.
And the fact that you can just print out a couple plastic parts in an hour, put it together and it functions with its double-action mechanic just like it's supposed to (unlike most prints which are either decorative or useful, but "inert" objects) is amazing.
https://makerworld.com/models/980402-otf-fidget-knife-over-engineered-and-upgradeable
honestly i'm surprised i've never seen those people selling 3d prints at flea markets sell some of these, i'm sure they'd do really well for at least a little while.