Seongju, this question is likely not ideal for stack overflow as"building a b2b site" means different things to different people. I encourage you to take this question to the Broadleaf Commerce Forums
I work for Broadleaf Commerce and the platform out of box is useful for some B2B scenarios.
The main benefits are that it support many advanced pricing concerns for B2B implementations via the programmable Dynamic Pricing API. In the enterprise edition this includes support for rule based price lists.
Also, the framework is designed for extension allowing you to add new attributes to the domain as well as customizing and adding new services.
At current, it does not have any functionality for often requested B2B features like quotes, purchase orders, and EDI integrations.
Regards,
Brian