Use WP Auctions to sell items only on a Buy it Now basis. Just set up your auction as you normally would and enter the Buy it Now price. Your buyers will see the form that specifies the price your item is available at and can purchase it right away.
Once they enter the information and click Buy Now, they are prompted to check their email for the payment details. As soon as you receive your payment you can ship out your product.
You can put up as many items as you like with this method and accept payments via PayPal.