Not sure if this would count as a "bug" but rather more of an exploit I found by accident, but can successfully replicate almost without fail. You can give someone a different item than what they are asking for if you simply hold it out of reach after showing it to them.
For example. A customer walks in and asks for a One Handed Hammer. If I have any other item on or near the front of store (2 shelves to the left, the one shelf to the right, or on the counter), if I hold the hammer out to him, but pull it back and out of their reach before they can take it, they will default to the next closest item. So although they are asking for the hammer they will take a great sword and pay for that item. Sorry if this isn't very clear it's late for me :). Great game and keep up the awesome work!