This occured in the UK, so not really relevant to the FAA.
In the US though, yes, the FAA would not have a problem with this. This plane would be registered as experimental, meaning you can install unapproved parts. It does have to be inspected, but that can be done by the builder (not the designer, just the guy who built that particular example), or an airplane mechanic.
I know quite a few experimentals with 3d printed parts (including my own). I don't know any where they are installed in a place where failure would result in a crash. Typically I see them used as convenience stuff, in my case I am using a 3d printed catch to hold my upward opening door while I load the plane. If it breaks in flight, I wouldn't even know. If it breaks on the ground, the door will close.
Flea markets at US airshows are not under FAA jurisdiction.
I have attended said airshow for decades and occasionally buy stuff in the flea market myself. Old used scrapyard parts, next to some inventor’s homemade jet engine, next to tons of raw materials of unknown provenance, next to ginsu knives and miracle frying pans. Here’s what it looks like on video. Wow, I missed those hand grenades for only $10 each, what a bargain.
