Hello!

I can't see anywhere a way to trade some outpost, but it is mentionned in the achievements and in the Lumeris traits that with the Lumeris we can sell outpost.

Can someone tell me about this? Thank you

It is done via diplomatic screen. You can sell your outpost to another empire.

I've yet to see the AI offer more dust for the colony than it cost to place, however, making the ability effectively useless.