Authorize.net is a payment gateway. All it is designed for is to act as the middleman between your site and the customer.
You can use Authorize.net a few ways. For example, let's say you have a simple web site that sells just one product. You could use Authorize.net's API to incorporate that into your site. When a user click on a link, it will send them to Authorize.net's secure form where they can fill out all their credit card information.
You can also incorporate Authorize.net into billing systems such as Modernbill or CMS systems like osCommerce.
I don't understand what you mean by "have to go with a reseller". That's entirely different - usually reseller refers to hosting plans.
HTH