I'm glad to read that you have resolved your situation. Something to remember, this won't be your last neg. People are ruthless on eGay and generally do not care about leaving negs without hesitation.
The messed up part is this guy could easily create new accounts and just blast your accounts with negs. Yea you will be able to get some removed or revised, but there will eventually be some that stick. I'm sure you know even a couple negs can seriously mess up the account, especially ones with lower feedback.
If you are selling digital items and they leave a neg, just report to eGay they left neg before you shipped.
You could always open a case with eGay and state that the statement was defamatory, him calling you a crook and it causing you to lose business. Always stick to facts and don't accuse or talk negatively when speaking with support. Request for a removal.
Or when speaking with someone who left you a neg, don't ever talk bad, threaten, or say negative things through eGay messages, they are usually less likely to revise it. Use the e-mail if you want to say something bad. You don't want to be reported all that stuff is recorded.
People also tend to use the same info with multiple accounts. Use the e-mail or other info like name, address to the big G. Maybe you can find a FB page or other personal info. See if you can "persuade" them to revise it. Through e-mail of course.