One possible issue with sign shops is that you can't tell how many items are in a sign shop chest. This is annoying for the buyers and can also be annoying for the market if people advertise prices that aren't real since they are not actually selling items of that kind.
Is there any way to indicate whether a chest has the advertised items in it or not?
Lynxy's proposed change to the signs might provide enough space:
[shopname]
24x Cobblestone
1 vT
Avail: 1728
This available balance of items on the 4th line would be overwritten by IDP whenever a shop sign is placed, so players can't provide fake data.