What items should I stock pile at the end of MM league that will be worth a lot more in ST. I'm new to the whole economy thing when it comes to playing til league's end and transferring stuff to ST.

mirrors

Metamorph goes to core so I doubt there's anything league specific to "stockpile".
Catalysts are always useful but not sure about their value.

nothing

Legion sprinters/ emblems

Honestly not a great idea as everyone has the same idea...Standard is where things go to die basically. You can make much more currency playing at the start of a league and transferring that to ST exalts or whatever.

League specific wise: 100's of Fertile and Prismatic catalysts might show you some return. It would probably be nice just to have them as well. People are gonna want them as soon as metamorph goes to standard.

Considering how the catalysts does affect crafting, I don’t see any reason to not invest in it. Although, at this point, I’m just too lazy lol.

dude, what the fuck is that avatar picture, please tag as NSFL