What do they do?!?!?!?!

Organic cucumbers do nothing, basically.

Cucumbers could be called a collector item, but do basically nothing like assilent said.
Only the pickled cucumber can give you a achievement if those matter to you.

Collector items, Do nothing
those also are part of an old event, where you had to destroy cucumbers

will it sell for like 100k in about a year if it goes out of stock or disappears

No, it won't, given the fact that there are loads of them all over the market, compared to something like highland tomatoes, which there are only three or four.

Mega is right here smurf, the more quantity the less it will be worth the longer you wait. Things like I think it was Skul Fuk's piano are very very rare because of the fact there is only four of them if I'm correct.