As Sikiro said, you must edit it on the npc.
If you want to edit the item default price, you must go to the item_db and edit the sell column.
For example:
// Structure of Database:
// ID,AegisName,Name,Type,Buy,Sell,Weight,ATK[:MATK],DEF,Range,Slots,Job,Class,Gender,Loc,wLV,eLV[:maxLevel],Refineable,View,{ Script },{ OnEquip_Script },{ OnUnequip_Script }
501,Red_Potion,Red Potion,0,50,,70,,,,,0xFFFFFFFF,63,2,,,,,,{ itemheal rand(45,65),0; },{},{}
to
501,Red_Potion,Red Potion,0,1,,70,,,,,0xFFFFFFFF,63,2,,,,,,{ itemheal rand(45,65),0; },{},{}
This means that every npc that sells apples on the server, will sell it for 50 -> 1z