Is it nickels where the older ones are worth more than 5 cents in the value of the metal? I read about people who acquire hundreds of dollars worth of nickels to sort out the older ones and hoard them. It's not legal to melt them down yet, but they are waiting for that day.
The opportunity cost of all that time and money is way more than their efforts could yield. But something tells me it's not about the money...
You can sell coins to dealers at "melt value" at any time you want to; this just means you're selling the coin for the value of the metal itself and now for the value of the coin, and usually done when you have a coin that is unable to be dated or is not a valuable coin (meaning the value of the precious metal outweighs the value of the coin itself).
Pennies pre-1982 mint date are mostly made from copper.
Nickels are made up of 75% copper and 25% nickel (from 1946-2015, but a possible change up to a stainless steel nickel is being discussed). Wartime nickels from 1942-1946 were made of 40% silver, so they are worth about $1 each.
And like I mentioned above, dimes, quarters and half dollars are almost pure silver before 1965, so they are always worth much more than face value.
There are a few reasons to collect: most just enjoy the hunt and collecting part; and there are folks that specifically collect high precious metal content as another aspect of their investment portfolios (having hard currency and/or gold or silver): and then we come to the folks that think they need to stockpile "valuable" coinage in case of zombie apocalypse/economic/government breakdown (and stockpiling food, toilet paper and guns/ammo are a given there as well).
I personally think old coins are beautiful and like thinking about how many places they've been and the people that might have handled them, but I do also enjoy the idea that I have a little precious metal in my possession (albeit very little - would NOT lock up a large amount of $ in coins/precious metals).
Just remembered - check out
http://cointhrill.com/ too. It's run by J Money, the guy that also runs Rockstar Finance and Budgets are Sexy. He's a big coin nerd too, and I believe he does trades on the site as well. :)