I'm new here, and thought I'd chime in with my opinion on SE friendly shopping carts.
I've owned a small hosting company for nearly ten years, and many of our customers have used a wide variety of shopping carts. While we offer osCommerce with our hosting plans (installed with one click from your control panel), few customers use it because of the steep learning curve. Most of our clients want to run their business and not live in front of a monitor. It may be free in terms of dollars, but you'll pay with your time and effort.
We have a sample osCommerce store you can place orders at to see what it looks like from the customer's perspective:
We are using MOD_REWRITE to make the url's SE friendly.
For customers that have a small budget for a shopping cart, we have sold and supported PDG Cart since 1997. We've had customers come from osCommerce, X-cart, and others for many reasons - It is secure, very easy to use for both the store owner and shopper, pages are totally changeable so they will match the rest of your site, and has many SE friendly attributes: static url's even though pages are dynamically created; you can insert desired text in title, meta, H1 tags or anywhere else; etc.
It runs on your own server, which has obvious advantages over hosted solutions like no recurring fees and total control; and you can install and test it for 30 days for free. We also offer toll free telephone support, even during the free trial.
We have a sample installation available you can use to see the pages generated, try the checkout, etc. The description page that explains more and links to the sample store is: