Sorry to disagree (slightly), but I dare say everyone on this board has some concern for their coin(s) worth.
Check out these prices. Some cents are bronze, most fairly recent are brass. https://rockawayrecycling.com/scrap-metal-prices/
This is where I got the price. Using that many decimal places (for mathematicians) ...it's close enough (for engineers). http://www.coinflation.com/coins/1909-1982-Lincoln-Cent-Penny-Value.html
Gold and silver are easier to isolate, due to their relatively high specific gravities, especially gold. Separating out copper is more difficult.
No. The 'O' mint mark your friend was talking about is from the New Orleans mint... which stopped making coins 107 years ago. I don't mean to sound harsh but have you ever heard of a google search? It's probably not a great idea to try to sell something before even knowing what it is (let alone rely on it to pay rent). If you type in something along the lines of "United states coins o mint mark" or "1979 o mint mark penny", you will easily find out that there is no such thing.
@RachelRuth , see what you started, in all the years I have been on here I have seldom had a thread to go over one page...
Are you kidding? The New Orleans mint stopped making coins over 100 years ago. Don't take any advice about coins from this friend. It's a D mint mark that has some post mint damage on it. Where's the "rolling on the floor laughing" emoticon here?
Of course, that means it's a rare gold error coin worth millions, because Dahlonega only minted gold coins, right?