You have different possibilities to integrate your products into a website. Chose between a "Buy now" button for single item purchase or a complete shopping cart, or a download form.
Sell single items
Create a "Buy now" button to sell a single item. Choose also this button if you like to sell a product with the affiliate programs Clickbank and PayDotCom
This type of integration is made to sell multiple products at a time. The Shopping-Cart is based on the standard PayPal Shopping-Cart.
With a download form you can offer your clients to download the product again after they purchased it.