Astute observers will note that this is very similar to the credit card situation: Overstock does not accept USD, they accept Visa. (and MasterCard. etc.)

If I go to Europe and buy something with my Visa debit card, my account is denominated in USD, but they do some forex and the merchant gets Euro.

If I go to Overstock and buy something with my Coinbase wallte, my account is denominated in BTC, but they do some trading and the merchant gets USD.