I am a lawyer but I didn't stay in a Holiday Inn Express last night.
Martin is 100% correct. The FMVSS applies solely to manufacturers of cars and car parts. It does not apply to vehicle owners. The only equipment on a vehicle that operates within the borders of the 50 US states that is currently federally unlawful for a vehicle owner to modify is emissions equipment (but even then, the EPA tends not to police/inspect individual owners except in select areas within certain states, and even then, they contract with the respective state governments to enforce such standards, which usually only consists of a visual/OBD inspection of equipment and/or exhaust gas analysis). Otherwise, as far as federal law is concerned, a vehicle owner is free to modify their car however they please. Whether or not certain modifications would meet a particular state's regulations / statutory schemes (such as bumper height, exterior illumination, braking systems, etc) would have to be checked against the regulations/standards of the state that one is registering the vehicle in for regular road usage, as the codes are all slightly different, although there is a fair amount of uniformity among the states.
Any modification made by the vehicle owner, the owner is implicitly accepting full and sole liability for such modification, as would any subsequent purchaser of the modified vehicle upon the purchase of such a vehicle. As long as Martin is accurately advertising the specifications and operability of his brake package, there would be no liability traceable back to him unless a part that he sold was defective (does not operate according to the advertised specifications) when it left his possession, and even then, he would be protected via the indemnity of the manufacturer of such product for the product defect claim.
Hope that clarifies things for everyone.