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Before jumping to any more conclusions, let's clear something up: Are both listings auctions, or is one of them a for-sale?
I assume that the listing on eBay is likely an auction, right? I mean, it could be a storefront "For Sale" item, but most likely it's an auction. If someone bids on the eBay auction and is the high bidder over any Reserve Price, then there is indeed a contract between the buyer and seller.
But is the Audiogon listing an Auction, or is it a For Sale? Big difference. If the Audiogon *auction* closes above any reserve price, then there is a contract to exchange, as well.
But you say that this item is "listed" in both places. What if it's an "auction" on eBay but a "For Sale" listing on Agon? No conflict, in that case. If there's an eBay win, the Agon listing can be cancelled. If it sells first on Agon, then the seller can cancel the eBay auction listing, although it's more difficult and a bit dubious to cancel any existing bids.
As a seller, I might certainly choose to *list* in multiple places, but I'd completely avoid running two different *auctions* concurrently. And I'd avoid buying from anyone who does this.