You can’t just sell an idea for a product. Unless the idea/product is in the public domain, a patent can be necessary to prove the seller owns the product/idea they wish to sell. In other words, why should someone buy something from you if you don’t have the title or deed saying you own the invention?
Design and utility patents give the owner a monopoly on making, using, and selling an invention in the USA. Design patents have a $265 minimum government filing fee with a 14 year life, and cover ornamental appearance (i.e. jewelry). Changing appearance by 10-15% can get around a design patent.
Utility patents have a minimum government filing fee of $533 with a 20 year life, and cover both function (i.e. how it works) and appearance. Inventions include mechanical devices (door knobs), electronics (circuitry), chemical recipes/formulas, and computer software. Provisional applications are not patents, but do allow one to use the patent pending status for a minimum $125 government fee. Provisional applications must be converted into a utility application within one year to obtain a patent. “Patent Pending” can only be used if one files a provisional, utility, or design application and it has not been abandoned.
Other fees necessary during the patent issuance and post issuance maintenance process add thousands of dollars to the cost. Patents must be applied for within one year of their public disclosure or it becomes too late and a patent will not be available.
Preliminary internet and library patent searches (UCF Library) can be performed for free. However, one cannot rely solely on their results, because their databases are limited. Professional searches done at the United States Patent Office in Washington, D.C. can be necessary to give a better determination of what is patentable. The USPTO recommends using a registered patent attorney or a patent agent in obtaining patent protection.