The issue is rarely the detector itself, it is what a missing one signals. A lender or insurer reads a basic safety item as a proxy for how well the rest of the home has been maintained, so a small deficiency found late can trigger a closer look at bigger systems too.
A pre-listing walkthrough that checks smoke and carbon monoxide detectors, along with anything a $400 to $600 home inspection typically flags, catches these issues while there is still time to fix them cheaply, instead of during the week a lawyer is trying to close on schedule.
Small, fixable items are the easiest delays to prevent and the most frustrating ones to discover at the last minute. Get your hand-reviewed valuation from Joel Dyck.