Mo NPC

Really it depends how NPC interaction is coded server-side, Nexon-wise. You can definitely enter numbers (Alcaster sells 1~100 of his items for example, ores can also be refined in bundles of up to around the same amount). As for opening shops, it may be possible but I think the only way private servers do it is a bit of a hack (the shop is set up for a different NPC, and the conversation opens that shop by pretending the client talked to that NPC).

afair numerical input at an NPC is a signed byte, thus limited to the range -128 to +127. Not so good for arrows, unless they sell larger bundles anyway (seriously, who buys 2 arrows? )

Still, if it works for Alcaster and Jane it should work for Mo. I think Nexon's programmers probably had some difficulty with how those two's scripts worked.