To be honest, I'd much rather buy and sell through the Trading Post area on TalkMagic; there's always loads of bargains to be had, new stuff on a weekly basis, and you have the added advantage of knowing who you're buying from.
Have tried a couple of dedicated Magic Auction Sites in the past, but never bought anything; most of the items are not much cheaper than full price, and many are listed by the dealers themselves.
So, no - it's TalkMagic all the way for me
Now....if there was a dedicated TM Auction site, for members only, that would possibly be a different matter (are you listening, Modsters?!

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