It was suggested to me to sell it for 25k, but as I don't see any in any other shops, only in galleries and things, I want to sell it higher...what should I do? :O

It only shows up if it's priced under 10k or 5k I believe. But you can sell them for around 100k at least. There are still people who are buying them for this amount to get the Achievement ^^

Thank you for your help! :D