the seller is only allowed to make fundamental changes to their auction when there are at least 12+ hours left and no bidders.
That's what this auction seller sounds like to me, he added BUY IT NOW, and probably changed the description too.
I've been eBaying daily for 2+ years, and have seen lots of things get pulled just before the end. Some of the sellers have told me that they changed their minds about the opening bid price (that they set it too low and weren't willing to "give it away"), and couldn't change it, so they cancelled the auction, only to relist it with all their changes. It's also the only legit way of having an auction run for more than 10 days. Very frustrating for those who snipe everything they buy.
In regard to the double bidding (bidding through eBay, then Sniping with AS), I do this a lot. Some sellers refuse to sell to buyers who kite in and snap up the prize without having established a bid at least several hours in advance. My practice is to low-ball my starting bid, and then Snipe a major bid nearer the end.
Recently, I've lost several bids to AS being "NOT IN TIME" so I've had to increase my snipe time to 15 seconds (from 7 seconds).
I hope some of this info has been informative....
Tired and shagged out after a long squawk....