Exchange rates are very poor here?

I don't think so. The renminbi is pegged to the usd, so exchange rates are standarised across the country.

Anyway, most people - local or otherwise - who want to buy dollars do so from the freelance moneychangers, who - i might add - are generally more than willing - nay - encourage you to do the transaction with them in a bank so that you may, at the bank, check and even swap the US bills they are offering you so that you can be assured of getting real ones.

In fact exchange rates are very good if ur earning rmb, because it is widely accepted that the rmb is way overvalued, and who knows how long that will last.