There are three scenarios I can think of off the top of my head:
1. Entrepreneurial Inventor: Here the inventor either has or raises seed capital to facilitate the production, marketing and sales efforts for a product he has secured a patent on. He can either build a company with the internal resources to handle the issues of bringing his product to market or he can outsource the responsibilities to vendors. Most vendors, if it appears they stand to land a lucrative contract will sign non-competition waivers that will not permit them to manufacture and sell the item on their own, work with other competitors, or divulge intellectual property to outside parties.
2. Inventor Sells to Corporation: The inventor sells off the rights to whatever he has developed for a percentage of the products sales, a lump sum, or a combination of the two. Other than collecting royalties he has nothing else to do with it. At this point the individual/corporation retaining the rights chooses how production will be handled and if outsourcing, will most likely require the production facility to agree to non-competition as mentioned above.
3. Management: An individual/corporation identifies a needed product and contracts someone to invent/design/engineer it with the stipulation that it will become the individuals/corporations exclusive property. The inventor/designer/engineer is paid a fee for services. Patent is obtained and outsourcing is done with a vendor willing to agree to the non-comp.
I’m sure that there are more examples and I’m equally sure we could find exceptions to the above but if, in the end, you chose reputable vendors who respect their legally binding agreements, and make sure all your bases are covered, you wont have any difficulties. Your biggest problem will most likely be other competitors who modify or slightly improve your design and patent it on their own.