Marketback Walker enters and dies as a state-based action due to having 0 toughness, triggering Mortuary and High-Society Hunter.
Resolve the Mortuary trigger, putting Marketback Walker from your graveyard onto the top of your library.
Resolve the High-Society Hunter trigger, causing you to draw a card, thus drawing Marketback Walker from your library.
Repeat.
This clever engine puts Mortuary and High-Society Hunter to work recycling Marketback Walker through a zero-toughness loop, generating an endless stream of death and leaves-the-battlefield triggers. Whenever Marketback Walker hits the graveyard with zero counters, Mortuary immediately tucks it safely onto the top of your library. From there, High-Society Hunter steps in to cash in on the creature's demise, letting you draw the very card you just tucked so you can cast it all over again.