Well I would reference the 3rd from the last picture in the series of pics on the e bay post? Short of someone spilling a liquid, somehow getting it sloshed up under the side there. I would guess they washed it before pics were taken and SOMETHING might be leaking? Well that and the rusted well itself may have something to do with it. and EXPERIENCE with the cars themselves? I mean really that far forward it did not come from the BACK window.
With the storage area as rusty as it appears, surface rust not rust holes I agree with wagonkiller. Windows are leaking, it might be the rear window or could be all three windows. But there is a leak, Paint on the tailgate doesn't look quite right. But maybe it's an illusion. Good looking car, wrong time of the year to sell it. For comparison sake I'm workin a deal in an identical spec'd 76 right now for far less money.
Still available? Apparently never sold last fall on eBay. http://davidsclassiccars.com/buick/...ded-clamshell-station-wagon-stationwagon.html
The ad is a bit confusing. He says to contact him if you have zero feedback, etc. So he just copied his ebay listing to this ad? It doesn't look like an auction site, just a for-sale site. No price given.