you go to ur seose file 2r2 Click Data>open NPSExtra> then pick a NPC like NPC 40 (shop bob) then all you do is type this is
[shop] Item=the Item ID Cost= how much you want it to cost Trade=0
*NOTE* EVERY NEW ITEM YOU PLACE INTO IT ADD A NUMBER TO THE TRADE!
Like:
[shop] Item=1 Cost=1 Trade=0
[shop] Item=2 Cost=1 Trade=1
[shop] Item=3 Cost=1 Trade=3
and soon and sofourth.
Hope it helped if u need anything else ask =D