Playing with my brother and had no issues until about 4 and 1/2 hours into the game. Bought a skillbook and went to buy a second one. I found that I couldn't.
I have enough money, but there isn't any deduction when I try, a clicking sound (not coin/cash sound when making a purchase), a normal closing circle around the book icon (usually to show you pressed "X" to buy) and the options at the bottom of the window change from "Sort, Buy, Show Details, and Close" to "Sort and Close." Moving the selection gives back normal options but repeats the same thing should I try to but them again.
This happens with any merchant, but only happens with skillbooks as testing it on something else lets me buy it. Player 2 seems unaffected (Otherwise we would have probably abandoned the save) and when reloading in single player with the same save, seems to not have any problems.
I think I was in the middle of buying the skillbooks when my brother transferred an item to me if that helps.
Please and thank you if anyone has a solution or if a dev takes notice of this weird bug.