Ecommerce platform possible?

> Would the programming language and the hardware support that?
Yes - although "3d fashion" would require loading a bunch of stills or a video.  no animated gifs.

> pay directly through Roku
I assume you mean pay through their roku device with the click of a button on their remote.  Yes, assuming you've somehow got their payment info linked up in advance, this is possible. 

If you mean "pay through roku" meaning the user would pay through the Roku commerce system then you face challenge in configuring your various products in Roku's backend to be payable codes that your channel obtains.  Then users could use their pre-registered Roku account and credit card and you'd theoretically eventually get paid out by Roku for payments your users made.
If you've already got billing system in place then you probably want to do the former because it will be easier to configure with an existing system already running without having to tie into Roku's back end, and you'll get to keep more of your money and probably receive it faster.

Per the current certification checklist --
https://sdkdocs.roku.com/download/attac ... 691&api=v2

"Channels with transactional content (SVOD, TVOD) are integrated with Roku Billing services" -- REQUIRED

For subscriptions and other options to pay you will need to work that out with Roku before publishing or distribute as a private channel.

well that's annoying.  But good to know - thanks destruk!
wonder why they don't publish the cert checklist as a webpage instead of this clunky method in a year as late as 2018

I agree Joe.  Although Roku wants to make their 20% off each in-app purchase as opposed to Stripe's and Paypal's 2.9% + $0.30 for the same charge.  I think there would still be some leeway if you sold prepaid credits on your website or offered subscriptions on other platforms that would port to Roku's app.