Are emuneration crystals actually sellable? I would expect that few people would want to buy them as the planet they will be applied to will most like be around MR 17x.

They are sellable, but pose serious situations. If a person bought one, they would have to trust you with their 17x

I believe that having them shamed on the forums, kicked from their legion (if not, then shame the entire legion), and being ZERO'd as long as possible would be enough to dissuade any potential planet stealing from occurring.

Good point. In other terms, yes.

Never got one, what does it do exactly?

there's another problem though....

since part of the procedure requires an abandon.... is it really worth it?
(note: if you have rings or moon(s) you'll lose those semi-permanant effects)

i'm staring at a planet right now... 37 buildings... i'm not going to waste mylarai extractors removing those so i can increase arti production rate by 1....

so that leaves me with only considering planets that have easily replacable buildings.... in which case you're looking at roughly 2 production for every 3 spaces... meaning you're already cutting your planet's possibly production to 2/3.... effectively turning a 18x into a 8x.... and... as rare are crystals are, who's gonna be selling them anyway?