2
Simple Paypal buttons are one way to go for a small shop. I personally would go for cubecart
http://cubecart.com/ or zencart
http://zencart.com/ as standalones with links from Xoops.
Another solution would be to use mals free e-commerce as a hosted cart, and use it from a content module.
http://www.mals-e.com/ Mal's service is used by many thousands, gives more choices regarding payment gateways than paypal alone.
If you want a bigger shop oscommerce is the most well known, there is a module somewhere if you do a search, I don't think it is currently being updated, not sure, but again, if using this solution I would personally go for the standalone version.