There are only 3 basic systems
http://www.mals-e.com/ or
http://www.romancart.com/Or paypal there are others but these are the simple ones for FREE.
Malls and roamcart are both very good and they have loads of examples for setting up you buying page. They have lots of features and can do all sorts of things like send out emails for you.
You basic problem is you cant accept any credit cards unless you have a payment gateway. Roman cart lets you use Paypal among others as a gateway as does MAlls. they both have videos and tutors online to help you and the forums are very quick replying
