Unless it can be proven beyond a shadow of a doubt that the customer saw the sign, simply having the sign alone may not be enough. A simple example - they could have a sign in the far back corner of the shop that says, "All firearms purchases will be marked up 150% without further notice." It's unlikely such a thing would hold up.
If there's absolutely no way one could miss it on their way in the door unless they chose to ignore it - you might be able to enforce it. If you have them sign on paper ... shame on them for not reading it before signing it.
If there's absolutely no way one could miss it on their way in the door unless they chose to ignore it - you might be able to enforce it. If you have them sign on paper ... shame on them for not reading it before signing it.