B2C marketplaces are common: Amazon, Airbnb, Instacart.
But what about B2B marketplaces? It's seems to me that since B2B consumers are usually repetitive consumers, they would simply refer to the original (re)sellers and deal with them directly.
Alibaba is the first to come to mind, as China's largest B2B platform (and by extension of China's exporting power, it's likely the largest such platform in the world—though Alibaba does list suppliers from around the globe). AliExpress and 1688 are similar, though more consumer and wholesale (Costco style volume), from the same company. Global Sources seems similar though I don't have experience with it, and there are many smaller specific-country-focused sites for finding manufacturers in a country for export goods.
I think the value of a marketplace is in both initial discovery (where Alibaba charges vendors to list products, in additional to transaction fees—so they win even if subsequent orders leave the platform) and in establishing trust (much like eBay, on a larger scale). The latter's especially important with global transactions where you may need a freight forwarder, customs documentation, and more.
Then, this discussion hit a similar topic—and the other marketplaces mentioned there were Handshake and B2BWave:
Looking for a better way to plan remote meetings across time zones, and keep up with events. What software is doing that best today?