I've seen loads of topics of people trying to sell things and saying I want x amount of dollars worth of BTC. Surely if someone is so keen on the idea of Bitcoin fiat shouldn't even be mentioned.
They should be prepared in certain instances to take a fiat loss for their beliefs.
It is reasonable and prudent to price and value items for sale in terms of something that is relative stable, e.g. USD. That way the seller does not have to change the price on a daily or even hourly basis.