Internet payment service provider, the party between you and the card companies. You can go it alone in this respect but it's not worth your time and the trouble, until you reach a very large scale.

Fraud control and compliance are the major problem areas in that respect.

Oh, I completely forgot to add: keep an eye on your tax obligations, especially VAT / Sales tax, it can make a huge difference where your corporation is located, the golden rule is do not incorporate in your target market or you'll likely be liable for a lot of sales tax.