Fishcart is perhaps the de-facto Open Source PHP cart. I don't think it has built in PayPal support, but it's in continuous active development and well might, or at least it might be easy to add the right code.
That said, I had a bit of trouble adding FishCart to an existing site (I didn't feel like rewriting the whole HTML/CSS code to fit around their headers, and the code for FishCart, while not overly complex, was IMHO, a tad illegible at first glance), so I hacked together a system of my own using some other tutorial scripts as a shell. I hope to have it online by April 1st. PHP is great for this stuff!!
