If you and I are trading, and you are selling 1 BTC for 240 dollars, then what is the difference between you sending me 1BTC, and me sending you CAD$240 from QS, verses me sending you CAD$240 from an alt? Even if the fact that what currency is being used is not documented properly (an escrow agreement should do this), it is still a scam in both instances. If this would be done as a direct trade, then it would be possible to weasel your way out of it, however if you were acting as an escrow, then your reputation as an escrow would be damaged because you did not properly gather all of the facts prior to advising that it is safe to send money
Let's use a not-so-obvious example to explain what I mean:
Let's say I'm selling 0.1 BTC for 28.3 dollars (note at the moment 283 is the average between BTC/USD and BTC/CAD). I send 0.1 BTC and you send 28.3 USD to the escrow. We didn't specify the currency and for some reason I think it's obvious it's USD because I charge a fee and you think it's obvious it's CAD because you charge a fee (neither wants to scam, we just disagree). If you are the escrow yourself then the escrow would also find it obvious it's CAD and he could just complete the deal.
I may start a scam accusation against both you and the escrow (which I don't know are the same person) and I may win or I may not. Only if I win the escrow risks his reputation and I may get the money back in an attempt of the escrow to keep his reputation. If I lose I just lose. And very probably I wouldn't even bother starting a scam accusation because of the small amount and because I'm aware it was my fault too for not specifying the currency on time.
If a real third party would have been used then he wouldn't be biased and I could trust much more his judgment. It's possible he decides it's CAD too; but he may also decide it's USD or he may not know and make an average or cancel the deal completely.
I'm just saying a real third party can act neutrally without any bias. If it's a neutral third party I'd feel much more comfortable trusting him. And I repeat I do not think this behavior deserves a trusted negative feedback, but I do prefer to use an escrow that never does this.