I don't know about legality of sales. You would really need to contact the companies about that and probably a lawyer. It would depend on what level of business you want to do. If you are just doing a few here and there I wouldn't worry too much.
For information go to
www.knifeforums.com and sign up to be a member. Once you are a member you can join the SOSAK (Secret Order of the Swiss Army Knife or something like that). You will find many examples of custom scales, custom blade combinations, and custom blades. There are several tutorials put together by members too. You will also find a little bit of information in their Outdoor and Survival forum also.