This is strictly my personal opinion, but I can't see any problem with selling a boat that you have built yourself. I don't see it as being any different than selling any other boat. However, I would add a couple of caveats. Pay the designer or company that sells the plans or kit for each boat you build.
Do not buy one set of plans and build multiple boats off of it. Most of the ones I have looked at state that you are authorized to build one boat with a given set of plans. Someone else has put an incredible amount of work into designing the boats and putting together the kits, and it is proper for them to ask to be compensated for their work. My other point is that at least some companies (I believe that CLC is one of them, but I'm sure there are others) license or authorize folks to advertise themselves as builders for their boats. If you are going to sell a lot of a particular model or brand of boat, it might be worth your time to get ok'd as an official/approved builder for that brand. My impression is that approved builders typically charge somewhat higher prices than us mere mortals, and one would think that a boat coming from someone like that would have to be pretty high quality, or the company wouldn't have authorized the person to build on its behalf.

Not sure if I'm on the mark on those points, but they are food for thought.