How to sell a file straight from a paypal buy link.

[gentleair]

I want tot be able to have people click on my buy link/button, and be able to download my file once they buy it, but I'm not sure how. 

I have a business account and everything. I just can't find out where to upload my file and link it to my buy link/button.

Someone help me as this is very frustrating. I've spent 2 days trying to figure this out.