That is a simple question but the answer could be huge.....!
I agree that this site is probably PHP driven and may well be hand-built. However, it is basically a database backed web application with a load of forms used to enter and retreive data from the back-end database.
There are several OPEN SOURCE products that can do this task for you, whereby all you do is install the website on your server and 'tune' all the variable information to your specifics. You would need to host your site on a server that uses PHP and MySQL (the database system). Have a look at www.osCommerce.com .
Alternatively, you can build a fairly professional looking website and database yourself, using any one of thousands of examples such as those available on other forums here. How are your database building skills? Coz that is where you need to start, by building your product and client database structure.....is this getting too far forward already?