Making a website?

I have a program that I want to sell online. Could I just build a website for it, put a paypal button on the website and sell it? And another question when someone buys the program, right after they have paid successfully, I want a link or a download box to put up so they can download it. But I need the link to expire tho, so it only will be available after each paid that is successfully ? Would this be legal?