I use osCommerce when I get a chance. The product is very solid with lots of features, SE spiderabiltiy is the only downfall for this product but they do have some good mod_rewrites that I'm going to test for in the future.

I'm currently running osCommerce on Cheap-Shades.com but I have no products yet. Working on marketing first before I invest $ for inventory for the site. But look forward for it to come online in next few months.

I do think that if you just needing a buy now button paypal is the easiest and very pratical for most uses.

I've also played around with phpshop and amazon web services.

But perfer using custom paypal buttons and osCommerce when I can.