For years I have kicked over going in the transformer business and I still might. But these are the issues as I see them: 1. I don't want to become an all around magnetics designer. I understand the basic concepts but the fact is it's a pretty arcane specialty. Such people exist, they can be hired if needed. You pay them by the hour or by the project. You own their work in the sense you have the only drawing and they agree not to give or sell the work to others, but, hey, anyone can tear a transformer down. As my great-grandmother used to say, that's just the how it is. Best defense is sell your product at a reasonable price, along with good service and customer relations.
2. I DEFINITELY do not want to sit and wind transformers. No offense ladies-that's women's work! They do it better than we can. (Until you get into large power distribution transformers with heavy bus bars and coils in the tens or hundreds of pounds.) I can set up a Gorman or a Universal and wind a simple item, but I'm never going to be good at it. Transformer winders, very good and experienced transformer winders, are affordable-it's hardly high paying work. If you don't believe me do a little digging and arrange a tour of a transformer plant if one is within range of you.
3. Things being what they are, people do not want new design transformers. They want old ones. My personal fetish is McIntosh, but that's another story. If you "clone" an old one from a recognized commercial amp or a catalog house part, it's going to be in demand vis-a-vis a new one. Many people want to build clones of known good amps: some people, astonishingly, have old amps they need a replacement part for.
4. "Intellectual Property". Is it wrong to copy and sell an old transformer? I don't know. Is it wrong to sell replacement bodies and necks for Fender guitars, aftermarket parts for Chevy engines or Ford rear ends, or aftermarket frames and slides for M1911 pistols? You can build "Fender" clone guitars from parts, Chevy engines, Ford rear ends, Colt pistols or AR-15 rifles....it's a long list. As long as integral features are not protected by patents, you did not misappropriate information from the designing company while working for them (trade secret), or cause people to wrongly think your product is made by or licensed from the originating company, (trade dress, logo, or trademark) you generally have a clear field.
One thing's for damn sure: Don't tell me you are the only one that can make a transformer with such and such characteristics and let me catch you putting a pair of cylinder heads on your race car the manufacturers copied the dims and bolt patterns for without paying GM (if it's a Chevy, likewise Ford, Mopar, etc..) or even getting their permission. I f***ing hate hypocrisy. Especially from people from Philly.
5. The kicker: capitalization. The thing that makes sense is getting an agreement with an existing winder that they will wind something only for you, at least for a while, or at least in a certain quantity. Transformer wind shops are not marketer-hucksters usually. That's why they aren't proactively winding a bunch of these things and advertising them.