(No, this guide isn't going to include many $$$!!!)
You can buy currency notes with many noughts on them quite cheaply on Ebay because-
1. The exchange rate is much smaller. This particularly applies to Zimbabwe, who have the trillion dollar note for about £20. But it can also apply to many other currencies, the Italian Lira is probably the most well known. Retired in 2002, it confusingly also had the £ symbol. There is also German marks between the First and Second World Wars, and many others.
2. Many Ebayers do not know that you can buy notes that cheaply, and suspect it to be a hoax, especially if it's of a currency from a country that is not wholey respectable... Many Ebayers would think it would go for a lot more, not that they're willing to pay that for it.
The downside of this means that you can buy currency quite cheaply but it is difficult to sell on Ebay.
Possibly also you can buy bulk lots of different currencies.
Current currencies are exchangable at any bank or post office-but if you want to stay popular, don't do too many...