Talk

I think that eventually the game buys your stuff from you via Wesley if no one else does.

It gets easier the longer you play as you can get more free gems from the other island.

I'm using the gems to expand production slots. Perhaps I should be saving them to expand the treehouse.
It's odd that some days stuff sells instantly at full price and other days I can't give it away. that's why I wonder whether they advertise it.
Maybe mine is being advertised to people who are asleep on the other side of the world .

I don't bother expanding production slots and just use the gems to speed things up if necessary. If your goods storage is full you can make space by harvesting from trees and then cutting any dead ones down so using your axes. Alternatively make other products that you don't necessarily need by using excess goods which will create space.