You can get the phone number with only your passport and some peso cash at any personal flow or claro store. Just remember to charge it with some money at some kiosk(not everyone takes it) or respective apps before you use it. You can't charge it in the store where you get the phone number.
Also, Mercado Libre will most likely ask you for a DNI for the account, I didn’t even bother trying to use it anyway... It’s a bit strange how so many things in Argentina requires DNI. Just a few days ago, when I ordered a McDonald's burger through Rappi, the app asked me for my DNI again, I just entered all zeros...it works.
Before the pandemic, I worked for almost two years in Shanghai, China and even in a totalitarian state, they didn’t even demand ID from foreigners this much. But anyway, every Argentine I’ve spoken to about this doesn’t care either, so fair enough with peace.