The X1 Carbon Gen 6 is from 2018. If it was used since then it doesn’t worth anything at all without battery replacement, maxed out RAM and a large SSD, even then, maybe 150 dollars max. If it is old stock it might still need a battery replacement and then I’d totally not pay more than 250 dollars.
Edit: I just checked PSREF: You’re limited to either 8 or 16 gigabytes of soldered RAM. You cannot upgrade or change it.