Here's how it works.
The gun is unregulated.
The cylinder is a gun part, not a receiver, so it's unregulated.
Federal regulations only cover what can cross state lines in commerce.
Now, once you put the things together, you are responsible for following local laws. For example, in some places, if you add an ejector rod and grind out a place to eject cartridges through the rear of the frame, legally, you have constructed a handgun. This is perfectly legal, however, if your locality (e.g. Clark County NV) requires registration, you are responsible for registering the newly-manufactured cartridge handgun. In some places, you may not need to do anything but drop in the cylinder for it be a cartridge handgun.
HOWEVER, the dealer is not selling a cartridge handgun if he sells the unregulated BP revolver to you, and, separately, sells a cylinder to you. You are then responsible for following any laws regarding the thing, including selling through an FFL across state lines. If he sells these things together, he is responsible, just like you would be.