If you have a single handle shower valve, have you ever pulled it out to start the water and noticed the entire valve moves with it? A lot of home inspectors pick this up during an inspection. It can certainly make a buyer apprehensive.
It probably stems from the fact that the piping and/or valve wasn’t strapped to the wall framing. The straps hold everything in place and so when you pull the knob, the valve stays in place. But what if the valve was never strapped in place?
The obvious answer is to open the wall and strap it. Of course then you have the burden of repairing the wall. You might be able to cut the wall open at the back side and repair drywall instead, or even install an access door. If you are lucky, the back of the wall is in a closet or some other inconspicuous place.
Aside from strapping the valve and piping, keep your fingers crossed. You can do a couple of things that might (and I emphasize “might”) minimize the movement of the valve. The first thing is to replace the cartridge with a new one. Old cartridges can get a little sticky, and installing a new cartridge will make the handle easy to move and put less stress on the valve. So this should minimize the movement. Additionally, you can try to squirt some expanding foam into the cavity around the valve. You will need to remove the escutcheon spray the foam in, an let it firm up. You may get lucky and it might work.