The best advice to anyone trying to sell an invention is: Make sure you do all you homework.
Is the product already in the market? Should you get a patent? A prototype and a well designed product will make this process that much easier and attract more attention.
No one really wants to take on all the hard work themselves to create your product so it is going to be very difficult to be able to sell your idea. The only people who may be interested in buying an invention may be companies who would save a lot of money from your idea, this would mean your idea would be a solution to a ongoing problem they have.