shop mod [REQ]

ok so im wanted to make a shop mod were
theres away to earn a currency
theirs a npc that you can right click
then a new gui comes up
each item/block has a specific price
if you have the right amount it can be purchased

also a revered system were you can sell item to the npc!
this would be legit!

hmm I will look into this

I know how... will add a tutorial later.

Well maybe what you could do for the money have it as like a copy of xp orbs then make it so mobs drop it or whatever then for the npcs why not take a look at the tale of kingdoms mod code for merchant just look for the line that says something like onentityrotclick

I can do everything except having it check if you have the right amount to purchase. I'm sure I could figure it out but I would love a tutorial on this.