This is a variation of MrCrayfish's "Best Trading System" as seen on YouTube. I basically modified his system so it doesn't have to be restocked and to look more like a shop being run by a villager. This is essentially a way to create villagers who sell what you want for the price you want, and who don't wander around all the time.
The schematic is currently configured to sell iron pickaxes for the price of 1 gold nugget each. To make it work, drop 1 or more gold nuggets into the hopper.
To change what the store sells, simply edit the command block by deleting "iron_pickaxe" and typing in whatever you like. To change the type of currency used, exchange the stack of 63 gold nuggets in the bottom hopper for 63 of the currency of your choice.