Im a bit of completionist regarding my spell book(gotta collect them all!), but being too greedy with bidding on "filler" spells on raids is a disadvantage to the rest of the raid force. I propose that completing "conqueror of (insert expansion)"-achievement, should unlock all rank 3 spells on the raid vendor for that expansion. That way you have the opportunity to buy the few spells you are missing with your raid currency, before your guild moves on to the next expansion. Do that retroactively for all expansions!