You may want to consider an Affiliate program such [url]www.regnow.com[/url. Not only do you get a shopping cart and buy links, but you also can also (pnly if you want) have your software sold on the sites of others. For a commission of course!
I now use Regnow almost exclusively on my site for selling the products of others. It works out very well. My own software sales have also picked up about 15-20% due to affiliates.