Very occasionally the shop system would grab a reference to your inventory a split second before it was actually ready, and once that happened it stayed broken for the rest of that play session, and every buy and sell attempt would silently fail no matter what you tried. Should be fully fixed now.
It came down to a few components interacting in a tricky way, possibly influenced by CPU performance. I'm currently doing a broader review to see if there are any similar timing or load-order issues lurking elsewhere.
Also made two changes to the sell panel in shops:
Selling a stacked item (like your Longsword x4) now shows a quantity selector, so you can sell multiple at once instead of one at a time.
Shops now only buy back items they actually sell themselves, makes sense that the blacksmith won't buy your health potions, for example.
The version number is now shown on the title screen, which is currently at version 0.601.