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.
Except you can use any bitcoin wallet so they accept bitcoin not coinbase. As absurd as it sounds when snapCard (a site that orders from other sites for you) first started they would only work with Coinbase. Enough customers complained that they had to offer a second option to accept bitcoin payments from anywhere.
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.