Check out other sites like btcjam, bitlend etc.. and other sites before going out and framing the idea. Your idea is good btw 
i think that is a new idia a website for loans using the bitcoin like warranty,its a simple idia, for exemplo if you need dollars, but you dont want to sale your bitcoins, you can put the bitcoins like warranty and get some dollars, later you can pay the loan with a interest and i give you back your bitcoins, how can i make a website for do it? thank you beforehand for your answers.