It's a question gets asked in various forms every few days (here's today's example!).
Someday somebody's going to come up with a workable approach for physical products (the digital scrapbookers are already doing it).
However, for physical products it's just a ton more complex. Questions such as "how should sales tax be calculated if your website located on state A sells something on behalf of a merchant in state B to a customer in state A, and who is responsible for accounting for the tax and paying it to the relevant tax authority?" prove to be real head scratchers.
Similarly, "how would you handle confirmations, payments and shipping if a customer brought two items from different suppliers at the same time?" just seems to get more rather than less convoluted as you work though possible solutions.