I want to sell software online and I need an easy to implement payment processing system. What I'm actually going to be selling is an activation code (one per purchase) that would activate the trial version of a product. I was about to use this one but I just found out that people without a paid email account (not hotmail or yahoo) can't process their orders, which I'm sure would discourage many, if not most, of the possible buyers.
I use PayPal for a shareware program I have been selling online since 2003. They have a number of ways you can integrate their system with your website, so that after a customer has clicked on a Buy button on your website, and then paid through PayPal (whether they have a PayPal account or not -- they can always use a credit card), then they get sent back to your site so you can give them an activation code (or download link in my case).
Since they have a number of ways to authenticate who you are beforehand (through bank accounts, credit cards etc.), I don't think they have a restriction on your email address.