I am tasked with making a custom ecommerce app that will populate purchase options from a table and keep track of orders, call paypal, etc. It will be pretty much like 'select options', 'enter payment info', 'confirm payment', 'setup account (now that payment is done)'. Another team here at the office is using MVC for a new project which this project will eventually be working with. The question is if an eCommerce app makes more sense to use ASP.NET MVC or regular Forms based? (will use SSL if that makes any difference)
Not only should you use ASP.NET MVC, you should have a look at Rob Connery's storefront application weblog series as a guide for some of your decision making. He goes through some of the technical/architectural decisions in an open series.
I'd go with MVC, personally. Both options are viable, to tell the truth. I just prefer the MVC model, it's lightweight, customizable, I know exactly what's happening (no server controls, yay!), have full control over the markup, integrating 3rd party libraries like jQuery is a breeze (even for Ajax - I used jQuery's Ajax with WebForms, but it was not as pleasant as in MVC). I don't know whether you tried MVC already, if yes - then you know how it works, if not - this might be a good opportunity for you to learn something new and fun.
An important factor in your decision should be support for 3rd party controls. Are there any that you would like to use for your application? Perhaps an out of the box solution such as a shopping cart? Note that MVC is relatively new and does not know the concept of server controls, so most of the existing solutions won't work or will require some hacks (it shiver me timbers when I think about putting WebForms stuff into MVC...). However with the concept of models, controllers and views it's fairly easy to develop your own solution. Have you done layered architecture in WebForms? If yes, then you'll feel at home, only everything will fill more at the right place. :)
Good luck!