After a certain point, money stops being currency and starts being collectible. France switched from the franc to the euro some years back and stopped converting francs in early 2012. You can exchange foreign currency for dollars if you have some left from a recent trip. If you discover an old cache of francs or guineas in your attic, you may be able to sell foreign currency as collectibles instead.
If you travel overseas regularly, there's no need to worry about exchanging leftover Canadian dollars or rupees. Keep them somewhere safe, and use them on your next trip. If your visit was a once-in-a-lifetime adventure, the best solution is to exchange the leftover currency. It's unlikely foreign coins currently in use are also collectible.
When you go through an international airport, you'll find currency booths that will convert whatever you're carrying back into American dollars. The next simplest solution is to call your bank. Some banks will let you deposit the money into your account and convert them in the process. Other banks will exchange customers' foreign money without a fee. If neither bank nor booth is an option, you can sell euros or pesos to a service such as Travelex that converts currency for you.
If the exchange rate is unfavorable and you don't need the money immediately, you can hang on to it until rates improve.
If you have an older currency that's no longer legal tender, don't stick it up on eBay without doing some research. For instance, coins and notes from before Britain switched to decimal currency are no longer legal tender, but some British banks still exchange them at face value. If exchanging currency isn't an option, look up the shillings or dinar in one of the paperback reference works on collectible money, or online. Some coins will be worth more than face value simply because they're made with silver or gold.
Other than the precious metal content, the market price depends on the money's rarity, whether collectors are interested and its physical condition. A pristine pound note that looks like it just came off the printer is worth more than one that's scuffed, dirty and has several tears. To appraise the money, you need to know the year it was issued, the country of issue and the condition. A professional dealer or appraiser may be able to help.
Once you know the value, you can offer the money to a dealer, to other collectors or sell via an auction. Unfortunately, the figures in the price guide are just an average of recent sales, rather than a guarantee. If you find someone who's interested in buying, they may not offer what you hope. It's up to you whether to accept or wait and hope for a better offer.