While leveling my BS, I noticed he had Ebony Boots in his inventory the other day. After 3-4 days in game at least ...I went back to him to get some leather strips and noticed he no longer had them, instead he had ebony gauntlets....so I waited 48 hrs in game and checked again, this time he had neither. Is this notmal? For his items for sale to change from day to day? And, will he have the boots again as I have never seen them anywhere else,, ( yet).

Yes. While vendors typically have a certain amount of fixed stock, the rest of it will be reset and randomized every two days.

Yep. It's nice - if you're looking for something in particular, you just rotate around the blacksmiths every couple of days, eventually you'll find it....

Not even that long. If you don't like what a merchant has you can exit the game and restart (to make sure, or just load an earlier save from before you entered the store (or city for outside vendors) and you will likely see a different stock for sale - which means you do NOT do this if you wanted to buy something and just needed to beef up your haggling by donating to a beggar or hitting a shrine of Dibella.