indeed, it seems that depending on your level, they have a fix list of skillbooks that they may have, every time you load a game, this skillbooks among this list are randomly chosen, so if you reload several times you might be able to buy every skillbooks on the seller list, but the list itself seems to change when you level up only

"-Oh that's fullmoon, cuttie cuttie sheep
-baaaaaaOOOOORGH"
***Sprotch***

Weresheeps will rule the world (At least one night every 29 days)