Named after Mephisto, an early player of these games, who popularised the technique. Basically, what you do is find a shop which sells what you want, and save your game before you go in for the first time after it restocks. Then you go in, and check their stock. If they don't have what you want, then reload the savegame and check again - the stock should be different. If they still have the same stock as they had the first time you entered, then you're going to have to wait until the shop restocks - I'm not sure how long this is - before this trick will work.
