Probably hands-down the most irritating metadata issue with Amazon is when sellers don't include quantity information properly - or even include quantity in the title when selling just one size - which I'm sure they do on purpose to make price per unit shopping more difficult.

Sometimes the way the items are listed will be different between one product and another made by the same company, or between Amazon's listing and a third party's listing for the same general product...and they each have different types of the item available. Say, Amazon will sell Ken's Socks in purple and blue, all sizes in purple but only small in blue...while Blankenship E

Things seem to get especially messy when you have a matrix of two 'flavors' (such as a sub-model and color, or unit size), and not all combinations were manufactured or available.

Even the UI isn't consistent. Sometimes it's a drop-down list, sometimes it's the rectangles.

Oh, and often the model/part number isn't entered...