There are quite a few regulations on use of rocket propellents to fly rockets, with several agencies having their fingers in the pot, including NFPA, ATF and FAA. I attached a link to a short reference overview, it is from the National Association of Rocketry or NAR organization.
There are two organizations that guide rocketeers NAR and TRA that provide certification guidelines for rockets including use of high power rocket motors that are reusable. Sale of these high power motors whether single use or reusable are restricted to those that have certification from NAR or TRA. Having had that certification in the past, I let it lapse, and top of mind there are a few things in here that would need quite a bit of research on legality.
Flying any high power rocket vertically, requires some FAA involvement for airspace clearance. I am going to guess even flying one horizontally say downrange could raise the ire of the FAA, whether it flys in controlled space or not. I think of this because the FAA now regulates UAS's or drones even though for the most part they never fly in controlled airspace.
Even though the ATF relaxed its interpretation of ammonium perchlorate (APCP) from an explosive , which was BTW only by order of a court, what I am not sure if the use of such in a RPzB 88mm would be interpreted as "sporting" and not interpreted as dangerous, even if only a dummy round. This would be something I would want a very clear understanding of before hand. Another link provided. https://www.atf.gov/...ellant/download
A bit of that changed SIG
Obama's Drone regulation silliness was overturned this Spring
Schumer & company have vowed to bring it before but as it stands it's dead as a regulation.
In another regulation FAA assumed control of all "airspace" over a foot above ground level. That was right before the Drone nonsense.
previously we had personal air space over private property, think it was 100', may have been even higher.
Allegedly this was done to protect Law Enforcement Drones.
The NAR like the AMA is a non governmental body...their rules are about meaningless in a Court Room. Plenty of hobby shops & outlets sell high power rocket motors cash & carry out the door. You DO NOT need NAR certification to purchase them any more than you need to be an AMA member to fly model aircraft.
reality is though unless you are out there posting You Tube videos doing rank idiot things with your sims FAA won't be getting involved.
Same is true of fools with Tannerite and ATFE.