Although selling an invention is a crucial process that likely should involve an attorney, I will suggest some general tips for you.
As others have already mentioned, the ability to sell your invention is often foiled by not having a patent for it. Investors typically take on a bunch of risks with startups and attractive inventions, but buying an unpatented product is a whole different type of risk. Thus, if you haven’t already, file a patent for your product. Also, stay away from a provisional patent as a potential buyer is not going to want to complete the laborsome process of seeking a non-provisional patent after acquiring your product.
You also need to spend ample time doing research on potential suitors. Think about your target audience and what you have that they don’t. Consider whether you are trying to sell your invention in its entirety or simply license the product to other companies. After figuring out who you want to approach and what you want to offer them then it’s time to put together a business plan. The U.S. Small Business Association provides a helpful guide on how to put a business plan together. However, as you can see, prepare to spend many hours doing this. I have found that inventors have a lot of trouble with this part because business management is not always in their tool box. Thus, hiring an attorney to assist you in creating a business plan and finding a buyer is something I recommend.