Key price

[🐸Flubs (Pepe)🐸]

I don't understand how key price works, if I go to backpack.tf or steam store or scrap.tf  keys have the same price
if I visit marketplace keys are worth around 20ref less than in backpack or the other site. so my question is why do everyone use the currency of the 1st 3 site and not marketplace? is there a difference in the key you can buy/trade ?
It all started cause i have a taunt that a person want and i went to some sites to see how much it was worth in keys and ref but i don't knwo which site should i use since if I consider the currency of the 3 sites i feel like i'm actually losing "Money"

[Gaia's Minion]

1 hour ago, drooomx said:

 

Marketplace has a different Key price because they deal/dealt with actual money and not metal or steam funds. So their price is lower because of that.