I have never met a person who was able to sell an idea for cash. I’ve also never heard of an investor making a decision exclusively by judging an idea. In most cases, having an idea is not even enough to land an appointment with an investor. You have to show something that proves you are capable of developing it: past success with other ideas, a working web application, or at least a minimum viable product.
Having a unique idea is way overrated. Elon Musk was not the one to invent the electric car – just take a look at how many electric cars were created before Tesla: http://en.wikipedia.org/wiki/History_of_the_electric_vehicle. And it’s not the only example. When Google was created, there were already a handful of search engines, and Facebook was launched when there were already at least a dozen other social networks. And vice versa, there’re lots of examples of companies who were the first to implement an idea but who lost: Friendster, Palm computer, and Netscape are just a few examples showing that a great idea is not necessarily a key to success.
Don’t get me wrong. I don’t encourage you to release your patents (like Elon Musk did). We need to draw a clear line between a startup idea and intellectual property. Using the example of Tesla: creating an electric car is an idea; the car’s design is intellectual property. Intellectual property (with few exceptions) should be protected; ideas don’t need protection.
It’s very likely your idea is not unique too. Just do some thorough research, and in most cases, you will find that someone has already implemented what you had in mind (in one form or another).
Don’t worry too much about competition. It’s more likely that there will be no one who cares about your idea, and that should be a much bigger concern of yours.