They can sell them because the manufacturer carries the liability for the quality of their product for the most part. If a car dealer sells you a new ford truck you do not hold the car dealer responsible if the truck is a lemon. You complain to them and in turn they get the manufacturer to fix the problem.

The trouble with black powder rifles is there are no set standards when they are produced or sold. No government regulations out there state, your barrel must withstand X PSI of pressure to be sold in the US. They do not even regulate the bore diameters as many have found out. They purchase a rifle and find they can not fit a certain.50 caliber projectile down the bore.

It is up to the manufacturer to go that extra distance to make better quality barrels and parts so if the rifle fails there is less of a chance someone is hurt.