Karma fades over time; how exactly this works is uncertain, although its speed is (roughly) inversely proportional to ethical magnitude, and varies somewhat between deities.
A handful of acts, such as hubris, are disapproved of by virtually all gods; no karmacist would knowingly touch the stuff, which is referred to as null karma.
Karmacists use the following abilities to manipulate karma:
Human Exchange. A karmacist can bestow karma on, or receive it from, a willing participant. Generally this will require a spoken acknowledgement of the deed, and it always requires specific consent and physical contact. The stronger a ka, the more difficult it is to acquire; it takes a mid-level karmacist to shrive a murder. There is no limit to how many karmae a person can carry, but they become more volatile as more of them are acquired, so the sensible karmacist pracises moderation.
A karmacist's own god(s) is obviously of very great importance to him, since this determines the value that karma will be stored at. Karmacists gain a little influence over karma's manifestation, so that bad karma is slightly less likely to crop up for them - but they know better than to rely on this.
Much karma-trading is done between karmacists; this is called the Deeds Market by karmacists and the Guilt Trade by everyone else. It's not very dynamic; prices don't change much, and a lot of travel has to be undertaken to turn a profit. Exchange between karmacists is easier than normal exchange.
Identification. This enables the karmacist to observe someone's karma. While it is said that the greatest karmacists can read the moral acts of anyone effortlessly, most karmacists have to work for it. A karmacist must pick out an individual ka before he can exchange it, so this is the first skill a karmacist learns.
With concentration, a novice karmacist can discern the general character of someone - whether their karma is good or bad on balance, but little else. A mid-level karmacist would be able to pick out most individual Significant Acts, and (with further concentration) identify their general classes. Details that aren't ethically important to the god of the individual in question will be obscure - but this doesn't mean another god won't care. Accurate descriptions of deeds, furnished by their owner, increase accuracy - this is the only way low-level karmacists can pick out individual ka at all.
Ai-Urthak and the Behemoth are both minor deities; Ai-Urthak commands his followers to fast on Wednesdays, whereas the Behemoth holds extravagant feast-rituals. Chris, a follower of Ai-Urthak, has recently broken his Wednesday fast, but luckily for him Karmacist Dave happens to be walking past and, squinting mightily, notices that that man has broken a holy fast. Thinking that that Act might be worth a little good karma from Behemoth, Dave comes to a quick arrangement with Chris and strolls off. He doesn't know - Chris didn't tell him - that the fast was broken with mangosteens, which are anathema to the Behemoth. If Dave trades the Act off to Behemoth with no further thought, he'll be saddled with a chunk of bad karma.Most karmacists have been burned like this at some point in their lives, so they usually insist on very complete confessions - and keep carefully-updated libraries detailing immortal tastes.
All this doesn't necessarily constitute a reliable history; karmacists are common enough that many people have gaps in the record. Karmacists can't spot such gaps.
Any karmacist can notice, through brief concentration, if karma is manifesting itself, or has done very recently; in the former case they can identify it as usual, but in the latter it's slightly more difficult.
Divine Exchange. The highest-level karmacists can trade kae directly to a god - generally in a fiddly ritual. If the god is pleased with the offering - the karma is good by its standards - it may bestow some undifferentiated karma on the karmacist. If the god isn't pleased, he'll instead bestow a good-sized chunk of Pissing Off the Gods null-karma.
Undifferentiated karma is not generally known about by non-karmacists; it is pure potential for action, undirected divine will. Its effects are completely unpredictable, and often extreme; even those who could access it generally prefer not to.
Distillation. This advanced and difficult technique lets karmacists divide a single action into its component ethical parts, paring off one at a time. (For instance, you might split the Fall into ingratitude, hubris, betrayal, attempt to corrupt, and so forth). This has only become possible in faiths with complicated, explicit theology - monotheism and a handful of the Etter city-gods.
Of course, the gods don't deign to communicate personally with karmacists, so all this is largely informed conjecture.
Some minor gods (notably the Etter ones) are area-specific and don't care a great deal about personal dedications; if you're in the area, then you're subject to the standards of that god, and formerly good karma may turn bad. Karmacists are well-advised to to their homework before setting out to such places.
Vessels are all other hired karma stores; they aren't used a great deal, because the risk that they'll spill the goods is higher.
Historically, most karmacists have been affiliated with the clergy; they usually form a seperate order or type of appointment, rather than being fully integrated into the heirarchy - though plenty of them rise to high ranks. For obvious reasons, karmacists are the foremost students of comparative theology, and (where possible) establish various libraries for the purpose. Admission is rarely free, and some libraries have branched out into other fields.
Karmacists are perforce flexible in their ethics. Most karmacists avoid breaking their own god's precepts most of the time, but many do so out of simple pragmatism; there are plenty of benign karmacists who look on the style as a merciful, forgiving way of bringing peace to people, but they rarely have the stomach for the bigger stuff that you need to play with in order to increase ability. Some faiths regard karmacy as a sort of mission - glorifying their god by purifying his people - but while karmacists often exploit this archetype, there are only a few who fill the role. Some are near-sociopathic; most are hardened and cynical. Legends abound about karmacists who did horrible things and then traded them off at a profit.
Minor karmacy is practised by a fairly wide array of people, from local clergy to witches. Confession is strongly associated with karmacy, however, so it's not much of a clandestine art.
School is going to be conditional on how the religious structure works out.