I'm a hacker, so this is how _I_ would be tempted to do it.
First off, if the guy uses a sniping service, you're probably screwed. Possible solutions at the end of my post.
Disclaimer:
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Don't do any of this. Period. It's just conjecture, and is a bad idea. All of it. I specifically discourage it.
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Method 1)
Send the guy an e-mail. Get him to respond. Often times, his IP address is included in the header, especially if the guy uses a freemail service. Wait until 45 seconds before the end of the auction. Flood away.
Method 2)
E-Mail virus, the day of the end of the auction. Hope for the best.
Method 3) Find out his ISP, get another computer on the same router. Wait until 30 seconds before the end of the auction, send some bad ARP packets, and have his packets routed into obvlion.
The below methods are ESPECIALLY not recommended, but should work regardless of if he uses a sniping service.
1) Offer him $50 to lose the auction.
2) Set up your own auction you know he will win. Get his address, put out a contract on his head.
3) Same as above, only have his computer trashed, or him kidnapped.
4) If he doesn't use a sniping service, find out his address, and blow up a nearby transformer 3 minutes prior to auction end.
The following should work if he uses a sniping service:
1) Bribe the sniping service to have "technical difficulties" during the last minute of the auction.
2) Hack into the sniping site, get his password, have him snipe a couple of really expensive cars, then change his password and email.
3) Figure out what the providers are for the auction sniping site, and flood the crap out of them for the last 3 minutes.
4) Flood ebay right after you make your snipe.
NOTE: DON'T DO THOSE. NO REALLY, DON'T.