It's one thing to sell a used bus, modified or not, as-is to a private party from a private party. Once you form a business specifically to modify busses for resale, you've put yourself into a whole new liability category.
I'm not referring to liability for what happens operating the vehicle, but for its roadworthiness, safety, equipment & standards of construction, etc. There's a reason that you frequently hear about recalls of vehicles. It's because their manufacturers are on the hook for certain safety & roadworthiness issues. Once you become a "manufacturer", you're on the hook, too. For that matter, your state may have certification requirements just to allow you to do that sort of work (think ambulances, fire trucks, handicap vehicles, etc.).