You would have to use a trusted escrow. You send the Bitcoin to an escrow and then he sends you the miner. You confirm that the miner has reached you and then you tell the escrow to release the funds.
How do we both agree on someone we can trust though.
You use someone trusted from here, like OgNasty. The list is here;
https://bitcointalk.org/index.php?topic=855778.0There are also actual escrow services in offices, but that sound a bit silly. You're probably better off turning the money into FIAT and buying with credit card or whatever. Beware though Ebay tend to be 50-100% overpriced.