Check out knifeforums.com, and the website in my signature. I don't think it would be a problem to sell blades for a SAK. However, changing the blade is a pain in the neck. You would need the proper tools, and selling blades for the average consumer to change ion there SAKs would lead to nothing but problems. For proof of this just join the SOSAK forum and do a search. You will see that modifying a SAK can be rather difficult.
Robert