few things come into play when selling to NPC (not LDoN merchants as they are on different table)
1. raw CHA. 130 is cap for merchant.
2. race
3. god
4. faction
#2 at times is very fickle and does not always play the roll properly. example is the ogre in FP will not sell very well to Ogres from Oggok for some odd reason.
with 130CHA, max faction, same race, and same god, you will have the chance to get best price for selling and buying.
as the above poster mentioned if you find an NPC on your faction even with lower CHA you will get better price then one you are on a lower faction table with.
example you are a DE SK, if you go home and sell in your guild hall to the DE merchants behind the counter you will get a better price then if you go up to PoK and sell there.
simple reason, you should be on a better faction table(ally if you did red wine quest) vs PoK were you will be just smack in the middle of the faction table with everyone else. same CHA, race, etc, faction will play the bigger roll there.