When I used to sell tons of stuff on eBay with the make-an-offer option, I’d hold off until the very end of the expiration in case someone offered more, or finally bought it outright. If not, I’d accept their offer in the final minutes. It worked in my favor on several occasions. Maybe your seller just forgot, because that happened to me as well.
As a seller, what I dislike about the offer option is if it doesn’t meet your reserve, you aren’t even notified someone made an offer. I believe it would help many sellers realize their perceived value is much higher if they received five offers on something lower than their reserve, but falling consistent with eachother. Then, maybe they’d lower their price just enough to entice someone to finally purchase the item.