It would depend on what type of coin it is and what country it is from.
It is very possibly a mintmark if it tucked away and not highly visible.
A "P" on predecimal British Sovereigns and Half-Sovereigns would indicate that it was minted at the Perth Mint.
It isn't a rare coin, isn't struck in gold and is only worth $1.
Rare Coin Wholesalers was created in 1990.
They are not rare.
You can buy them at coin dealers and coin shows.
Just the year does not make a coin rare.
No, they are not rare.
If your coin does not have a mint mark then it means it was minted at the Philadelphia mint. These coins are usually not as rare. However recently P mint marks were added to coins to indicate that they were made at the Philadelphia mint.
It means that the coin is rare. For example a 1909-S VDB cent is a key date. This coin is rare.
Determine how many were minted, when and where and the numismatic grade and, last but not least, the availability of the coin for purchase
No.
No and they will never be rare.
It does not have to be old at all. An error in the minting of a coin may make it rare. What may turn out to be a rare coin may have been minted in 2012, but nobody has found it and come forward with it yet.