That'll help lock people into the Facebook ecosystem!
Today we’re beginning to test a new feature to help businesses drive sales through Facebook in News Feed and on Pages. With this feature, people on desktop or mobile can click the “Buy” call-to-action button on ads and Page posts to purchase a product directly from a business, without leaving Facebook.[url=https://www.facebook.com/business/news/Discover-and-Buy-Products-on-Facebook-Test]Facebook Tests "Buy" Button In News Feed Ads[/url]
Reading about this, I think we can assume that sellers will be required to use FB's merchant service for credit card processing.
We’ve built this feature with privacy in mind, and have taken steps to help make the payment experience safe and secure. None of the credit or debit card information people share with Facebook when completing a transaction will be shared with other advertisers, and people can select whether or not they’d like to save payment information for future purchases. ...
That seems to be the new biz model for these online services. Squarespace, for example, gives you no choice on their ecommerce platform--it's Stripe. Which has a good deal on processing–if you don't mind waiting 30 days for your money. Most Squarespace customers will not care.
This is one reason why FB says this:
The current test is limited to a few small and medium-sized businesses in the US.