I start by taking any personal attachment the account and throw it out. If im attached I keep it, its just easier, you will never get what its worth to you emotionally.
Then I check what similar is actually going for. Low end accounts are a dime a dozen. They are easy there is a lot to compare to. Now when you have a rare account or high value account it gets tricky. The line gets blurred.
Is someone going to pay what I feel its worth?
Is there a big market for this specific valued account? If you are in the high end market are a lot of buyers looking in your price range? Or just a few?
If its an account that has a lot of alt characters or games in it, do they add value or hurt it? too much can hurt an account too.
Did I buy high or low? How much do you have into the account monetarily speaking and time.
How secure is it? All your accounts should be secure, but original owner and low number owner accounts are worth a lot more, did you buy it directly from original owner? Is that person well repped?
Copy's comment on how hungry am I can go multiple ways. Like hungry, as in "how much do I want to make on this account?" aka money hungry. or Hungry as in, how much money do I need to make on this account. For instance if I have a bill I need to pay I will sometimes price account at a specific price to use the money to pay that bill or part of it.
You have to look at the economy of the game and its economy here mostly. Whats in demand? do accounts sell? what do they go for? how long do they take to sell? Why do some accounts not sell for a long time?
Usually accounts do not sell because of high price or person will not negotiate. Always buffer price for negotiations. If you want 100 for it try asking 125. Always more then the least you are willing to take.
Now some of accounts sit on sale for a while. I do not just settle unless I really need the money. People love to buy low and low ball. If keys are $60.00 do not offer me less then that if the account has a lot to it and worth a chunk of change. I know everyone wants a deal but sometimes its insult. Sometimes people offer exactly what you are looking for or slightly more.
An account is truly worth what someone will pay for it.