Seriously though, why aren't you?  "You can't sell somebody else's stuff without permission" is just basic science; if the little details that have been used are true and this is a future Earth, even a post-apocalyptic one, what reason would they have to change that when society rebuilds?

But she specifically mentioned the legality of the sale, when it shouldn't have been.

If she's so smart, how'd she not realize that the sale wasn't legal
because the seller didn't have authority to make the transaction?

I just want to know if the writers missed it, or if they thought of it and had Bubblegum miss it, but that sale was not legal.  Ash didn't have the authority to make the transaction, so Marceline was well within her rights to recover the property, and having a princess backing her would surely help.