What had Valkryst suggested will also remove arena points from prize and make items free for sale (costing only gold if they have buy prize set, likely they don't).
I suppose you don't know what extended costs are for and/or how they work. Once again:
NPC vendors have items in their vendor list assigned in world.npc_vendor table
That table contains column extended cost. It can be set either in DB or ingame by optional arguments in .npc add item command. Extended costs are in ItemExtendedCost.dbc
If you still don't know what the hell to do with information I gave you, use google for finding more information about npc_vendor and ItemExtendedCost.dbc and read getting starded tutorials.
