A genie grants you the ability to have infinite amount of one item, what is it?

Lithium.

We need it for all kinds of things, including cell phones and electric cars, and lithium mines are a nightmare.

Not only will I manipulate the market so my endless supply of lithium shuts down the worst of the mines (are you buying from one of those mines? call today for my “becoming a better person” discount!), I’ll partner up with any corporation that figures out an ethical and environmentally-friendly way to mine it (acquire one unit of lithium in an ethical way and get the second unit free!).

I’ll get rich before flooding the market tanks the value, and I’ll be providing something that’s a need, so it’ll be worth it to keep going after I’m not getting rich off of it anymore. I’ll constantly make a surplus that’s just my personal property, and have my lawyers set up something that supplies that stash to ethical corporations after I die.

Technology marches on and lithium stops being as vital as it is?

Sweet. I’m already rich, I did my bit to make the world better when I could, and now I can relax with the knowledge that supplying the world with the stuff that makes technology work is Someone Else’s Problem.

Lego clone troopers

Twenty dollar bills.

Genie lamps

solid gold coins.

Melt them down as needed and move them around. I doubt one person alone can crash the gold market if I just move it based on my needs.