You'll have trouble selling it without getting the transmission fixed, so I would recommend that you take it to a transmission specialist and at least get it assessed and quoted so you know what you're up for. I doubt that the trade would be interested in it, unless you could perhaps convince a Citroen dealer to trade it in. Other than that you could try selling it to a specialist Citroen wrecker. Whatever you do, you will take a bath on it if you don't fix the transmission.

Kia's great reputation here goes back more than a decade, Citroen is still challenging for respect from mainstream car buyers. Saying that one brand is more trustworthy does not remotely imply that the other is not.

It would appear the gas is leaking from the struts, and quite rapidly it would seem given that they are in effect running out of gas in 12,000 km. Go back to the dealer and Citroen and try to negotiate a new set of struts under the warranty, or alternatively have the struts regassed independently. I suggest the latter would probably fix your problem.