It seems that payment processing is the biggest issue here. People are very reluctant (rightfully so) to give up their financial information, so I think it is a

It seems that payment processing is the biggest issue here. People are very reluctant (rightfully so) to give up their financial information, so I think it is a good idea to start with an existing service like PayPal or amazon FPS. Next, I would select frameworks based on the development environment you're most familiar with. I might choose java/spring/hibernate/struts2 or even raw jdbc and servlets because I've spent so long in the java world, but many in this community would probably lean towards RoR or similar. I think it's almost always easier to go with what you know.