Sign [free] works when you stand block in front of it you click anywhere opposite, next to, the same height or higher then sign but not as it should. Also only owner can use them properly, [buy] [sell] can't be used by anyone else then owner.

Did you give all the players the relevant permissions for each sign?

Closing due to inactivity. If you need further assistance, feel free to respond to this issue and we'll reopen it.