Android Auction House/Player Shops

Hello Unity community,

I am about to start a project involving a life simulation game like Animal Crossing and Fantasy Life targeted towards Android. In the game, I want to have it so that players can acquire items and then post it either to an auction house or a player shop so that other users can buy them with their in-game currency that they have earned.

I am new to multiplayer systems development and I am not sure where I would start in performing a task like this, I would love if I could get someone to point me in the right direction.