I would say one of the best things you could do is find a Web site with the type of system you want to put into place, and then figure out how you can do the same.  If you want an Amazon.com type interface, then you will be looking at some of the bigger shopping cart systems.  If you just want to sell a couple of things, you might look at just using a few PayPal links. 

I would say, first things first it is most important to figure out your own business model.