ok so Greninja has set spawn times dawn-dusk and early morning's best to have a Gardevoir with trace and fast balls as Greninja is a very fast Pokemon also super rod or better is needed and remember 4 blooks deep in a swamp :D if you need more info tag me hope this helps
morning if you use /chestshop then make the shop using that you can set the price stock then bind the item to the chest :) if you need more help just ask