Probably your best bet would be to get an addon that shows you current/recent auction prices on your realm, either in item tooltips or even just in the auction house when you're putting an item up for auction.
Going through every single item in the game and trying to guess whether it should be included in a list of "gear that people think looks cool enough to transmog" seems like an enormous amount of time and effort for very little benefit. How often does this really happen, anyway, and what did you really lose? You looted the item for free, so the only thing you "lost" was something you didn't have in the first place anyway. Plus, you probably made someone's day by giving them a cool transmog skin for way less than they thought they were going to have to pay.
