Back-up offers
“If the seller accepted someone else’s offer, should I put in a back-up offer?”
Sure, why not? It doesn’t hurt you at all, if the terms of the back-up are written correctly.
Your back-up offer should require the seller to give you written notice that the first contract is cancelled and that prior to this written notice, you can cancel your back-up offer anytime.
Don’t sit around and wait for the cancellation, which may never happen. Keep looking for houses, and if you find another one, then cancel your back-up.
I had a client as the 3rd back-up offer. The first two didn’t come through, for whatever reason, and my clients bought the house.