My default currency is CAD (Canada, eh). I bought inventory in USD using a CAD credit card. I will be selling it online in USD. A few questions...

You will need to record the transactions in Canadian Dollars.

If you haven't enable the multicurrency function in QuickBooks Edit > Preferences >Multiple Currencies - Company Preferences tab.

QuickBooks will do the exchange rate conversions for you, all you will need to do is enter the appropriate exchange rates in the field on the lower right of the transaction entry screens (Bill, Invoices, Write-Cheques & etc). The rate will be reported on your credit card statement.

You will need to set-up a new vendor that uses USD for it's currency. Then enter the purchase using the 'Enter Credit Card Charges' as usual. Nothing else will change when it comes to making credit card entries or payments.

For sales in USD, if tracking each customer you will need to create a new customer as a USD currency customer (definitely for any credit sales - Invoices). Or you cans use a common customer (ie. USD internet sales), or even leave the Customer/Job field blank,  entering in the customers specific details in the Name/Address and Ship to fields on the 'Sales Order' enter screen. Enter the daily exchange rate.http://www.bankofcanada.ca/rates/exchange/daily-converter/