A seller agrees to the terms of the service, in this case Audiogon, prior to listing. Conducting transactions outside of Audiogon after using Audiogon services to sell your item is 1) against the terms of usage and could/should get you banned 2) unethical no matter how you look at it 3) cheats Audiogon out of the funds that keep this site open and working for us.
If the seller wants to avoid any fees, he/she is free to start their own trading exchange.
I would tell the seller to complete the sale on Audiogon or back out. Otherwise you have zero recourse should he turn out to be a fraudster. These rules are in place for everyone’s protection. If he gets snippy or threatens to write bad feedback, contact Audiogon support with the documentation trail and they will help you. They are very nice people.
Good luck.