The only way to make this a non-risk situation is to pay him cash in person when you pick it up in person. (Don't send thousands on paypal to someone with zero feedback)
and before even buying it, contact officials at Wynn and give them the serial number and state it is on ebay, to see if it is still a cashable chip. (if you don't state it is on ebay they may think YOU are the thief and say sure come on down!)