You can always use PayPal's shopping cart. I couldn't find a shopping cart I like, so I'm writing my own, using PayPal as the payment processor. What can I say, I have way too much time on my hands 😃
It all comes down to how badly you need one. If you need it right here and now, then go with a pre-made one. If you don't really need one right now, can spare a couple of months, and want a major project to work on, then you can write one yourself.