Are you sure his old inventory is the same? Most NPCs reset their inventory after about 10 to 15 minutes real time (depending on the NPC). As you found, NPCs can hang on to some of the stuff that you sell them, which stays at the top of their inventory list. Perhaps once they collect a certain amount of stuff they stop resetting the remainder of their inventory?
In any case, if you buy back all the stuff he is hanging on to (or trade it for charms, etc) he should start resetting his inventory again. Alternately, you can trade more with Blake, or wait until you get to the Dark Forest (if you have not been there yet) and trade with the smith and his assistant in the Dwarven halls.
