Assuming you won't be pumping out thousands and thousands of individual items that someone would have to scroll through or search through (same with blog items).
It sounds like you will need to use Zapier, and possibly another third party file uploading service like UploadThingy.com or UploadCare.com (nicer). But this is why Webflow sucks. You pay a ridiculous amount for web hosting, AND you pay for the CMS, but there is no way to upload anything besides image files. And even then, there is no way to upload image files from the front-end.
I did manage to find this but it will only work if you plan to avoid the CMS and export your website.
And what's better is that Webflow has increased the pricing. So MAYBE just MAYBE we will get file storage support in the next major update. It's sad that I have to wait and rely and hope for this. I would like to be entirely dependent on Webflow but I can't. Tough times... tough times...
Anyway, in simple terms: you cannot create a website just like you described without spending ridiculous amounts of money to rely on third party solutions that only large corporations should have to pay.
The same scenario goes for billing customers, except much better. You will have to use a third party solution again. Foxycart.io (domain) or Shopify are your only options unless you go the less automated route and use Paypal and send a download link.
Your best bet is to design the website in Webflow and create the functionality with an exported version and suffer without a CMS, unless you want to invest days into connecting the website with a CMS of your choice. Webflow really doesn't make things much easier than it preaches. Other static site generators can provide similar results in a faster timeframe and within a CMS like Wordpress.
So pay unnecessary amounts of money, export your site and suffer, or use Wordpress and Divi 3.0 and florish on your decision.
I just really wish Webflow were more transparent. Someone only gives negative feedback when they really want your product/service. And I WANT Webflow, but the company is not transparent and barely seems to be listening to the community, so I'm complaining. Anything on the forum that is a request is basically, "oh, that didn't quite make it into this build but maybe next time" or "that is something we have in mind in the future" or some other statement that makes you feel like they really want to include it. I'm sure they would want to include it somehow, but the lack of transparency inhibits us from knowing exactly where Webflow is and what to expect from it. And therefore it reduces our ability to create sites for clients and ourselves in expectancy of upcoming features. And by "upcoming" it's nice if we get progress updates at the very least.