I'm not entirely sure the Greyhawk Adventures line sliding into negatives necessarily reflects returns from distributors/retailers. It might, but then why the little boom right after it?
There was a GenCon in that time period, I think during that little upturn, where I bought my copy for something like $4.00 at the TSR booth. The story they told me was that they had found some that had been poorly inventoried (lost in a warehouse or something) and so they were fixing to move them all at a low price.
I don't believe it had anything to do with 2e having just appeared. The Greyhawk Adventures hardcover has a blurb printed right on the cover that it's 2e compatible.