Usually the seller takes on short-term price fluctuation risk. Buyer initiates purchase, and the seller gives them a window of x minutes to send y btc to an ad

Usually the seller takes on short-term price fluctuation risk. Buyer initiates purchase, and the seller gives them a window of x minutes to send y btc to an address, where the price of the service is denominated in btc as of the trading price at that moment. If at least 1 transaction doesn't show up on the btc network in the x minutes, that transaction times out. If at least 1 does, then the seller waits for w confirmations to take place before the payment is recognized as valid, where w is set to their tolerance. I've seen as low as 3 and as high as 7.