-create an iCloud account just for the sale. This way you can set an iCloud lock password for the buyer. Once the buyer receives the item, I make sure they email me that they have the phone and that it is as described. Then I give them the iCloud unlock pass. When creating this iCloud account, don't use the phone to create, use a computer because phones are limited to starting a few iCloud accounts before it does not allow anymore. Also doing this you can set the phone to send last known location. If your extra paranoid, put in a cheap prepaid data sim to track along the way incase it can't jump on to the buyers wifi witout a password.
I've been selling and buying on eBay since 1998 - I was one of the initial beta accounts. I'm not a professional seller, but have amassed (425) 100% positive feedback. Point is, I'm experienced in both the buying and selling arenas, and with some high dollar items.
If I purchased an iPhone on eBay, and received it with some ridiculous caveat that I had to unlock it with a particular password only to be provided by the seller, I'd immediately open a SNAD (significantly not as described) claim with eBay, obtain a full refund, and send the phone back to the seller at the seller's expense. The buyer should have ZERO obligation to contact the seller in any way once the item is received, and I suspect this is mentioned somewhere in eBay's terms of service, and could get your account terminated. eBay provides buyer and seller protection - there's no need for you to add an additional layer. People - don't do this.