If someone contacted you out of the blue then you would be a fool to sell at $12. The odds of that being the highest they would ever offer is close to zero. However, if the name has been floating around on the boards and you have listed it for a similarly low price, then this $12 might be close to the best you can do.
If the buyer truly wants the name at $12, then why not try to get a bit more? It is highly unlikely that you will insult them so much that they will then turn around, change their mind, and then not want it for $12 after they turn down your counter offer (it sometimes happens, though).
Each situation is different. People are saying sell without having all of the information. It doesn't work that way. There are lots of variables that you didn't mention. The buyer told you the least amount he would like to pay. It is your responsibility (and challenge) to get more. The risk is always in blowing the sale, but this isn't the type of name where you will offend someone with a reasonable counter offer.
You have the name, so you are in control. It might be worth it to you in the long run if you take this negotiation and treat it like a lesson. The most this lesson will cost you is $12, and that is if you blow the sale. So, that is the worst-case scenario. The best deal is a quick deal, but you can definitely counter-offer a few times back and forth without dragging it out, and you will gain more than $12 worth of knowledge no matter what happens.
Those who recommend that you "SELL" without having any other information are not doing you any favors. It is not like this is a worthless name where someone offered you 3K. This is $12, and without any other information, telling someone who is looking for help to "SELL" is about the worst advice you could ever get.
*the exceptional businesses of our esteemed moderators