A friend of mine has asked me to write this on his behalf because he wants to create a shop that is inside of his TARDIS. He's using item frames with the items he's selling with their name as the price they are at.
His iissue is that item frames can be taken out of the frames and thus be stolen by other players even if his TARDIS perms are set to nobody.
He says that he discovered this because someone wanted to buy from his shop and they accidentally broke an item out the item frame. Then he asked them to put it back and break it again and they were even able to take it.