Regardless of date and denomination, all error coins need to be seen to assess a value. Take it to a coin dealer.
1c unless its a mint error. If you think the coin is an error, check with your local coin dealer
Mint error coins can fetch a premium over the normal coins. It is unlikely that you coin is worth more than £5 - £10 despite the mint error. Listing the coin on the eBay auction site with a good description and photograph should help the coin find its true value.
If the coin is an Mint Error it needs to be seen to determine what type of error and it's value if any, so take it to a coin dealer for an assessment.
A coin such as this needs to be seen to determine if it a true mint error or not. I suggest you take the coin to a coin shop and have it appraised.
What do you mean "double faced" you mean like a doubled die error? Or do you mean it has heads on both sides? If it has heads on both sides it is not a mint product but rather a "magician's coin" and is privately made and pretty much worthless. If it is a doubled die error, please provide the year and mint-mark of the coin so we can better identify if it is a known error and how much it is worth.
The date and mint mark are on the the edge of the coin
Take it to a coin dealer to see if it has any value. It's likely not a Mint error but has been mutilated.
Nothing
The mint mark by itself isn't worth much. It has to be on a coin. If you do have a 2001 coin with a D mint mark, it's far too new to have any special value unless it's a high-quality uncirculated coin.
$680 to $27,500 depending upon the mint mark and the condition of the coin.
A typical Mint State coin is about $50.00
It depends on the condition of the coin. A poorly conditioned coin is worth about $18. A perfectly conditioned coin is worth about $23,500.