Well, I have done extensive shopping on Alibaba's sister site, Taobao. Taobao is China's eBay, and unless you have connections in China, you would need to go through a Taobao buyer to act as a middleman to get you your goods from China, they usually take 18-20% cut, if I remember right.
As my understanding goes, Alibaba is similiar to Taobao, but devoted to wholesale, and transactions are pretty loose because deals are worked out rather than set in stone. So, imagine a version of eBay where side deals are actually encouraged. The part that I am unsure about is payments, but if they are on the same payment system as Taobao (Alipay), then purchasing would be difficult. You would either need to pay through corporate means of payment, such as bank wiring, or you would need an account with the the Bank of China. Although, the sellers themselves may or may not have a different means of making the purchase available to US consumers.