I've thought of that before, and don't see why it's terribly different from selling potions. Any low level hero with a blur potion can whiz straight through ANY low level quest, yet that isn't frowned upon that I know of, since the player/character payed for something that gave him an outstanding advantage.
If selling "buffs" is a problem, why are NPC's allowed to do so? I think this should be reconsidered.
There is some definate RP potential for clerics here, where they can prove the might of their god through themselves to newer PCs. I certainly see the "exploit-ish" side to it, but perhaps a less firm ruling would be in line, such as keeping it to only a buff or two, or a combined spell level of 4 or somesuch.