One of the issue that you may face (unless you are manually going to create every link for payment) is that any website that "pulls" info onto a page (like stock, sizes, colours, prices, tax etc) will generally get this information from a database...and iWeb isn't a database driven system at all. It means that most likely, if you do want to use a shop system, you'll have to use it slightly "standalone" to the rest of your site and that, if you are hosting on .Mac, you won't have much luck.
Keep in mind that what vectran suggests with things like Joomla or Drupal are database driven websites that can include a shop systems...not just shop systems. And what do I mean by shop systems? Well this one is getting some good buzz at the moment...
http://www.openfreeway.org/
There are many more also.....but basically if you go that way, you're going to be looking at setting up hosting, mySQL (database) use etc, to build a shop site that you set up standard info for various items, then it is just a matter of adding individual infor for each item...the system (once set up properly) deals with display, shopping cart, some aspects of the transaction etc.
The easiest (but not the most pleasant) way would be like you see on the main page of this site. Go to the bottom right and click on the link to buy a coffee mug...hey, in fact, buy one if you like

...and you'll see it take you to PayPal. The link that did all this was all created within the PayPal website...I then created the image, text etc to appear on this site. Not a nice way to do it, but one you could do if you only had a few items to sell.
Phil