Well, Canada and the USA aren't part of that, that's for sure. Every restaurant, Walmart, mini mart, gas station, hell, even furniture stores will take the opposite countries currency happily. Typically at a loss for the buyer. I've ended up doing this a few times when I've been short on cash and not a single time have they asked for any information at all, ever. In fact, all the currency conversions I've done at professional institutions, such as large currency conversion chains... never once been asked for info.
Perhaps in border towns, but in most parts of the USA, stores will only accept American currency and will look at you suspiciously if you try to use foreign currency - even Canadian.
I used my Canadian credit card there successfully more than once, effectively making the retailer accept Canadian dollars.
That's not how foreign credit card transactions typically work - the merchant gets paid in his local currency, and your bank exchanges that currency for your local currency (usually charging you a foreign transaction fee or a more expensive exchange rate).
I've seen some merchants in heavy tourist areas let the customer decide which of a few currencies the transaction is charged in when you make the transaction, but that's not the norm.