Looks like you've hit the shop bug (which was present in the original game). It happens if you've been in a shop and you then leave and quit the game without saving (either manually saving, or by leaving the map to load another map), or if your party dies before saving after leaving the shop.

There is no easy fix :(, you will need to either play without that shop or start over from scratch (you could exit to the main map, and then restart the game and try using the reset utility if you want to keep your characters).

Actually, there is apparently a way to fix it if you're technically-minded, HERE is where it is described in full (I've never tried it myself).

In the future, the best advice is to always save your game after exiting a shop. If you accidentally sell an item you didn't want to, closing the game while still in the shop apparently won't activate the bug.