Some major online B2B marketplaces in the US include:

1- Alibaba.com
2- Thomasnet.com
3- Global Sources
4- TradeKey
5- DHgate
6- Wholesale Central
7- EC21
8- Kinnek
9- IndiaMART (also operates in the US market)
10- eBay Business & Industrial section (for certain B2B transactions)

These platforms cater to various industries and sectors, offering a wide range of products and services for B2B transactions.