How do I justify an offer without turning the conversation into an argument?

Justify the offer with evidence the seller can recognize: recent comparable listings, specific findings on this vehicle, and the cost those findings imply.

Stay calm and specific. A number without a reason invites pushback; a reason without a number is not an offer.

If the seller provides new evidence, update the number in the open. That is stronger than defending a stale figure.

Walking away remains available. An offer you can defend is also an offer you can leave behind.