Go to "Sales". You will see a list of transactions there, and there will be links with each transaction where you can purchase insurance.
The only problem is that there is a delay in getting closed transactions to show up. If the transaction isn't there, just file a support case. Give them as much info as you can about the sale - item number, buyer name, price, title, etc.
But, in this busy time with the move and all, it is hard to say if they will be able to help you as soon as you need it. I know they are swamped right now.
Good luck!