I've found through years of experience in working with inventors that even if they don't find the invention in the market, there can be several reasons for this. 

    First, the invention may have been patented before, but for a variety of reasons, either never made it to market, or was taken off the market.

     Second, even if the invention was not previously patented, there are already one or more close variations of it that have been patented, such that it is unlikely the present invention will be able to pass the "obviousness" standard required of new patents.

     The only way to look at these possibilities is to do a patent search of previously issued or published applications for patents. 

     In order to do the search, a person needs to know how the United States Patent and Trademark Office stores these records on their database.  It is not as simple as doing a text search on the database. 

   The other goal achieved by a patent search is that if the inventor then submits an application for registration of patent, the United States Patent Office requires as a part of any such application that the applicant disclose the "prior art" patents that are closest to the subject invention. 

     Thus, the patent search will find those prior art patents that are closest to the application and those patents are then made part of the disclosure in the application.