Link hidden: Login to view this is a web-shop I used for a clients web-site once. Works well and is free. Easy to connect within your own web page.
You can also look into pro pay as a payment portal, loads better then paypal and easy to set up and use.
Ecrater allows you to create your own webstore that you can link to your site, but I believe you need to have a payment portal set up to link with them. ie google check out, propay, paypal....
Hope this helps a little bit