The reason is simple. The seller wants the most he can get for his book. He is selling it via auction style and the results have yielded sub par results on the past 2 occasions. Sooooo....time to game the system.
The seller bids on their own book up a point where they want to maximize their price, hoping, in turn, that someone actually outbids their own shilled bid and will actually pay it.
If not, no worries, they will just simply "win" the book in their dummy account, but will then contact eBay and claim non-paying bidder.....or better yet, a cancellation.
They get their fees back and it doesn't cost them a thing. It only costs the legitimate eBay bidders time and frustration for bidding on a rigged auction from the get go.
Even better is that eBay does nothing at all and lets it happen.
As for GPA analysis, I have no idea, but it Is actually logged as a sale on eBay, so I would think it has some sort of impact.