Oscommerce is an opensource store with ways to interace with paypal and other payment gateways.
My hosting company and im sure many others have a on click install for OsCommerce through their control panel.
If you want a more custom solution, you can work with PayPal developers console and write your own gateway. Its not too hard if you are familar with PHP.
If you want, shoot me a PM and I can setup a demo account so you can play with the software.