I would not disregard the live listings entirely. Completed transactions necessarily describe an earlier group, while current buyer financing and seller motivation may have changed. Compare both sets rather than choosing one.
For the live cohort, record the first advertised price, first...
The relationship is easier to preserve when the rationale is brief. A long defence of every cost can sound as though the decision is already final. Show a few relevant comparisons, acknowledge the tenant’s record, and leave space for a counterproposal.