Question.

ughm,well first of all i don't know if it is the right section for that but leaving that aside there are certain factors i personally consider when selling or buying a character because it goes both ways :
-gear
-level achieved(the level of the character)
-Skill points left or if none are left if it is full farm with all needed skills and extra ones for fun
-If it comes with gold or not
-Silk items ( premiums and devil spirits active are a plus ) , i dont really care about avatars my self but i search for chars that have lucky avatars or the gold to buy one
Also take in consideration all the nifty things you are personally looking for like:name,job level,guild maybe and so on .
After all of that asses the market ,see what chars compare to yours and at what prices and based on that you can go lower if you feel it is needed or higher.
Hope my 2 cents help.
Cheers.