Seller accounts can use different combinations of payment processors so it makes it impossible to provide a global shopping cart that would work across the site in a lot of scenarios. We do have some plans to make bulk purchases a bit easier when buying multiple games from single developers though.
I know this is from 4 years ago, but you realize that right now, anything with $1 for a regular selling price means it can basically just never be on sale, right? Paypal and card processors won't let you buy things for less than a dollar, so with no cart, there's no way to buy these unless you buy them for $1. Now, for one thing, that would be fine, but if I want to buy 20 things that are normally a dollar, that's now significant. You guys have to come up with something, either letting us bundle by seller or payment processor or whatever, we need something so we can actually utilize sales on smaller things.