I am using version 1.7.10-1.4.4.1177-server

I have a simple shop setup with 3 signs. Yet the logic isn't working. Look at the video below. I have 3 signs, each with item frames above. Sign 1 below item frame 1 sells item from frame 2. Sign 2 below item frame 2, sells item from item frame 3. Item frame 3 sign says the frame is already being used...

https://youtu.be/alFuA_CR4zE

I can give you access to server if you want to debug.

Looks like if you create the shop (first click) before you place the frame but you have another frame on the wall it will use that one instead.

@olee I am not sure if this is an intended feature or a bug

@securitypedant If you want to place shop signs next to each other, you should initialize (first click on the sign as user with sufficient rights) them one after another starting from one side.

This is because FE will look around the sign to find any unassigned item frame.
If there are more than one, it can get confused.
IIRC it will look first above the sign and then continue to look at the sides.

Just remove the item from the item frames (which will destroy the shop but keep the sign) and then put in one item, click sign, put in next item, click next sign etc.

TL;DR: It's not a bug, it's a feature.