It actually sounds like what you're trying to build is a lot like PowerStore, which you might want to take a look into. Its a pre-built solution that you would just install on your server and run through an installation wizard to get working.
From your starting point, I would recommend using the recordset on that page to insert different information into the add to cart button for eCart. You would want to put the add to cart button in the repeat region for the search results. To be honest, this is such an open ended question and there's no set best way to go about setting up the store. You could set up security assist so that you can store which order is for which user. Then you would also want to set up Universal email after setting up eCart to notify the user that the order has gone through and also to alert your store administrator that an order has been placed.