How does AliBaba make money when buyers and suppliers can use it for free?
From what I can see there is a premium membership i.e Gold Supplier but there is the ability to use AliBaba as a buyer and a supplier for free.
You will find that the most important thing for a buyer to find a supplier in Alibaba is trust. To gain trust, supplier attains the Gold Supplier status from Alibaba, that's how traditionally Alibaba make their money. Nowadays, Alibaba makes money in a lot of ways.
Nowayads it's not all about the ''visible money''. And it's also not only about Alibaba. The same way a lot of big online stores work like taobao and tmall. That's the business model.