That is a long delay, I personally think it's an unreasonable time to ask to delay posting, and it would result in a late delivery dispatch mark on your account.
Even though you would have messages from the buyer asking for this delay, not sure if eBay Customer Services would / could remove the bad mark .
Personally, I would send the buyer a polite message explaining the above would be detrimental to your account, and you would need to cancel the item which fully refunds the buyer, and relist when they are able to be home to receive the item.
Cancelling citing Buyer Requests to Cancel, will not affect your seller performance.
@fanli-303