A multi-offer is a one-shot tender. A backup offer ties you up. A letter of intent is not a contract.
When more than one buyer offers at once, the agent runs a multi-offer process where everyone submits their best terms by a deadline. You get one go. A backup offer sits behind an accepted offer and binds you while you keep looking. A letter of intent sets out terms without creating a contract, and can be useful or dangerous depending on how it is written.
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