Design, Build New Website and promote with SEO - Long Term

We are looking to build a new ecommerce website to promote a product we already sell. You must build the site from scratch with secure a secure check out system. The site must be able to be updated via a data base and with a simple user interface. Customers must be able to search for the item either by manufactures PN or by the intended use. We sell mounting sytems for homes, cars, trucks and vans. A person may not know the part number, but they would put in the make year and model of their truck and we would pull all items to work with it. The site must be SEO friendly. We can supply the data base. We will pay for the webdevelopement and we will also pay you for the SEO on a per sale basis. We have tried SEO in the past with an upfront fee only to pay the fee and see no results. We can see paying you the builder a fee of $2 per order. We could easily sell 20 to 30 orders a day, that is a nice $1200 a month.