Volunteering on a film set can be the solution to how to sell a movie script, as it will give you the opportunity to network with a lot of people over the course of the production, but there are a couple of things that you need to keep in mind if you are going to try to implement this strategy.
First and foremost is that, depending on where you live, there may not be any movies shooting in the exact town that you live in (which is the situation for the vast majority of towns, by the way.) If that is the case, you can look up the film board or film information office of the biggest town that is closest to where you live and try to find out if there are any upcoming productions that are coming to film in the general area. Paperwork has to be filed ahead of time with the state before the production starts filming, so if you have a film office in that city, someone should be able to inform you of what the status of any upcoming productions that are coming to film in the area are.
Also, you should keep in mind that while there is a chance you could be asked to do something related to writing, there are any number of other things that you could be asked to do as well, which include, but are not limited to, the following:
1. Performing in the film (a lot of writers are also performers, especially in the comedy world)
2. Being a part of craft services (dealing with food for people on set)
3. Location scouting for the film (will depend on how familiar you are with the area)
4. Providing and applying makeup (either regular or SFX type) for the cast
5. Working as a direct assistant to the producer or director
6. Providing a script polish of the current shooting script (great way in terms of learning, at least partially, how to sell a movie script)
7. Being a part of the technical crew (AD, Gaffer, etc.)
Should you find yourself with the opportunity to work on a film set, keep in mind that how you present yourself is everything in these situations. If you remain ready, willing, and able to work and don’t come in with an attitude, people higher up on the ladder will take note of that and ask you to work with them again on future projects (again, which could involve learning how to sell a movie script), which will only deepen your friendship with them, which increases your odds of being able to pitch them on your project.