This is Outdated Please visit this tutorial instead for Version 1.13 Minecraft!

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First of all Turn on Cheats

Then type
scoreboard objectives add dummy Name of Virtual Money

You can change dummy to totalKillCount or playerKillCount

then type
scoreboard objectives setdisplay sidebar Name of Virtual Money


then depending on your objective (dummytotalKillCountplayerKillCount)
it will add 1 to your money

Then how to make the Shop
First make a Wall about 1x3x3
then make a button

make a command block with this command in there
/scoreboard players remove @p[r=5,score_a_min=b] a b
a= Name of Virtual Money
b= How much


then make another one command block with this in it
/give @p[r=5,score_a_min=b] c d
a= Name of Virtual Money
b= How much
c= what item (ex: minecraft:diamond_sword)
d= How many items (If you don't put any number it will auto-choose 1


Then hook both of the command blocks in the button
Then if you have enough Money for the thing you will buy it will give you it :D

Notes:

Might add a new player in the sidebar: @p[r=5,score_a_min=b], If it did remove "r=5," on the command blocks

You will not receive any items if your Money isn't enough

Gallery: