This reminds me last time I was in a bar in the States, I was showing off some NZ currency to some local patrons and when I showed them the gold $1 coin, I told them they were extremley rare and worth quite a bit because they are gold. I ended up selling two NZ $1 coins for $20US each. Not a bad exchange rate. Next time I go over I'm not going to exchange my money at the banks, I'm going to exchange my money at the bars.
That sounds like how my uncle added to the family coin collection. He had a dairy cum tearooms in Ngaruawahia and used to accept any currency at face value for purchases from tourist on the way through. Sometimes he lost on the deal (often he didn't) but he didn't want make a profit, he wanted the new note or coin for his collection and Ngaruawahia is a pretty small town.