go to npc.xini and then find the npc u want to sell something (has to be a shop npc...)
example for the npc to sell something so u cld buy it.
/* Shop Bob (Malone, Weapons #1) */
[npc15]
{
[shop] Item=21 Sell=100
example for the npc to buy something from you.
/* Shop Bob (Malone, Weapons #1) */
[npc15]
{
[shop] Item=21 Buy=100