CPA Tracking solution

Hi, We have comparison website where Merchants list their product. We compare products and send user to their respected Merchant's website. We are looking for CPA solution. 1. If we want to track that sale made on Merchant website how can we do. The merchant are registered with us directly and not with any affiliate website. 2. our website is in ASP.net so is any ready made script available or can any one code for us. 3. Is this thing possible through google analytics ? If you have done CPA tracking solution before we would like to see some example of it. If you think you are genius and can do please let us know and we can go from there. We can only pay for the solution. We will not pay for half solution or for trying. Thanks