The site just provides the trading "platform". If a fiat is selected it will display the BTC equivalent for either conduct the trade in BTC, if is one of the ways of payment accepted by the seller or buyer, or BTC promotion purposes.
So if the seller wants Paypal USD and the buyer wants BTC, you will perform the clearing trade on their behalf. Sounds pretty cool.
No, if the seller doesn't want BTC, simply won't select it when creating the deal.
And this platform is meant to allow both end business; selling or buying. If you want to buy something, let's say a pair of pants, you start it by saying your maximum budget for it is 10 USD. A offers to sell it at 9, B at 8... reason why I said "the seller OR the buyer", depends on what end the deal was started.
As it goes maybe I'll give your idea some thought, but that would depend on a stable BTC... otherwise I would be playing Russian roulette by intermediate such trade.