What kind of item? Is it a digital item (e.g. ebook, mp3, video...) with needs for secure digital delivery?
Is there a lot of items to sell?
What kind of volume do you expect?
Those questions all make a difference. For example, if you have a lot of items you want a database driven dynamic shopping cart...
If you have only a few you can get away with using plain paypal forms to sell things...
If you need to tie purchase to delivery somehow it gets complicated, if you are processing orders manually then any solution works...
If you don't have volume in sales a merchant account may be prohibitive and this influences your cart choice..
Anywho here's the basics:
Paypal - You can make static pages with forms with pricing and get their address etc. This is best for very few items. There is also a simple paypal cart you can use.
osCommerce
http://www.oscommerce.com/ - This is the most popular open-source shopping cart and it works with a variety of payment methods.
There are, of course, a whole lot of other options, especially if you want to spend money. Some services even exist that you may like, services that combine the hosting, shopping cart, payment processing and sometimes even delivery. Yahoo (
http://smallbusiness.yahoo.com/merchant/ )is the leader here. This kind of solution isn't appealing to those who know how to do all of those facets well as we can inevitably do it ourselves better than Yahoo will do it for us, but it brings a certain convenience for those less deeply involved in web development.
If you want to look for other options search for "shopping cart" and you'll see a lot of ads and results with a lot of options.