It seems when you sell something, it's the sale amount in gold, not the number of items sold, that determines how much you improve your Speech skill. This is far less tedious than the Mercantile skill in Oblivion, where it's the number of items that counts, forcing you to tediously sell gazillion items. Here in Skyrim, all I need is look for expensive items to sell. I can even enchant it to make it more expensive with some useless enchantments, so I can train Speech and Enchanting at the same time. Having to sell fewer items, I also have to carry fewer items and not hurt my encumberance. So it's win situation all around. This is the kind of subtle change that brings about major adjustments in gameplay (in a good way) that I wish Skyrim had more of.