-> Because some users are returning users already informed what the product is. For them is convenient to see button right away.
-> Because if you place that button decent and it does not damage your design, its not bad to have it after multipole sections so users have it always available.
"Buy now" or any other Call-To-Action (CTA) button are usually the main focus for the companies website. These buttons are how they convert sales.
As such, standard practice tells us that CTA buttons should be placed at the top so that it's one of the first things the user notices.
"Oh hey, that's where I press if I want to buy the product, good to know,...
Interesting question, a couple of points:
For example, are users likely to be more suspicious of typing in the unfamiliar fb.me link as opposed to a facebook.com one?
This is definitely possible, I'm fairly tech-savvy (software developer by trade) and I'm not 100% sure that fb.me is legit. You should ideally test this with your users, are they put off? ...