I put some jars of rennet in my stall. 6 were q31, 4 were q27. However, the stall shows them all as being q33, which is the quality of the jar itself. This isn't very useful, as most people are more interested in the quality of a jar's contents. Generally, items in stalls should display same value as they do in a character's inventory.
Unrelated stall bug:
The other day I saw a bucket of water for sale in one of the Wayneville stands, and I couldn't see it's quality.