ID 15 was wrong:
Could not sell +1 leather armour to John. Reason: noinventory
Leather Armour is #14:
Sold leather armour to John for 11 gold
Sold leather armour to Aaron for 11 gold
I will send them on their quest a little later.
Good thing I didn't have any leather + 1 then. Tom does the game currently track the gold each shop has? Could we display it on the shop inventory page?