Do I need to have auth on for 3 days in order to buy character from trade?

Hi!

I've never donated before and I'm interested in buying a character from trade. I added the 2 factor auth just now, so do I need to wait 3 days before donating & buying? Or is it only for items?

You need it on for three full days for anything involving trading coins with other players.

Alright but if I donate, after having 3 days of auth, I can use the coins immediately? Or is there a wait time for coins aswell?

If you already have had it on for 72 hours (not three numerical days, but 72 hours from when you turned it on) you're free to use coins however you like. It's not per donation, it's a one time thing. Only if you turn 2fa off for some reason you would need to wait again the next time you reactivated it.