Which US city is the best base for an international e-commerce business?

Your office could be anywhere. If you are going to be dealing with China a lot via communication (email, phone, fax), it would make sense to minimize the time difference and an office on the West Coast will help. If you plan to look after entire US, then perhaps Texas, etc. would be a good place to setup (lower operating cost than say Chicago, NYC, SF), Austin, Dallas, Houston are all good candidates.

You want to check out the cost of living, and operating amongst other things. If you slightly move out of the main city center, operating costs go down dramatically. You also want to understand the relationship of where (which State you are incorporated under) and by operating business in a specific State, County and City what 'extra' expenses in form of taxes, etc. might be applicable to you.

With the current downturn in the economy, housing is relatively more affordable and office space is plenty to choose from if you negotiate with the property owner.

Also, if your resellers would need to travel to your US warehouse, or your office, you need to take into account what that means as far as air travel, etc. is concerned. An office on either one side of the States (East Coast or West Coast) can mean a higher travelling expense for your resellers who may be on the opposite end.