Insurance for unoccupied coop apartment while selling it?

Try Chubb. They're expensive, but have extremely good coverage. When I sold a condo I insisted on staying in it one week after closing to give me a chance to move all my stuff. I was able to get insurance for that week without a problem. If you need an agent I use Lee Insurance in Lower Manhattan phone 212.962.2232 Website:
www.leeins.com/main.html

Check your building's insurance coverage first. If you've removed all of your personal belongings, the need for a personal coverage policy might be unnecessary as most coops master policy will cover for fire and any other damages to the apt. You generally only need a personal policy to cover property.

As someone who has been sued for no reason except to get money, I suggest that you also check for personal liability insurance. My condo insurance covers not just property and personal belongings but also personal liability. Make sure you're covered for personal liability (i.e., someone viewing the apartment trips and sues you).

Though they don't offer renters' insurance directly, Geico has an insurance brokerage that works with most of the major insurance companies, and they were even able to find me insurance for a co-op on Fire Island - not the easiest thing in the world to do. Highly recommended.