I go abroad a lot, and I'm trying to think of a situation where I'd want to sell US currency. What with credit cards, ATM's and a few travelers checks for deep protection, I never have more than a few bucks US on me--and that's usually just to get me through the airport when I get home.
Presume waiters and stewards on ships still accept the small pictures. The days seem numbered even for cash tips, now that the lines are starting to convert to putting it on the account--a far better way to go.
AR