Thinking about it, it's almost cheating to bypass the quantity prompt. If the "challenge" of increasing your mercantile skill is all about clicking buttons in menus, I'm cutting out the biggest chunk of it when I ctrl-click and sell items one at a time a lot faster. Would I even have the patience to sell them one at a time if I didn't have TQP? I'm pretty sure I just sold entire stacks before I got it, though I don't know if I knew how mercantile worked back then. I feel so guilty now! If I didn't cheat, my mercantile skill would be at, like, 20 instead of, um... 30. I've totally ruined my game!
You know, that reminds me of something I was thinking about the other day that's totally unrelated to mercantile, except that it's another skill that you can completely ignore (actually mercantile has a much more tangible benefit): Personality. I came across this on UESP:
With a sufficiently high Personality, normally hostile beings (e.g. bandits, rats, dremora, etc.) will no longer attack you on sight, because their disposition is so high it counters their high aggressions. Your personality needs to be roughly 180 (depending upon Fame and other factors) for Animals to become non-aggressive; higher Personality is necessary for other enemies.
My idea is to make a mod that reduces the point at which enemies will yield or refuse to attack you, so if you spend your game getting personality up to 100, by the time you've maxed it out a good chunk of the enemies won't bother you. Doesn't that make a lot of sense since as it stands I have a personality of 20 and can just cast a charm spell to get information out of everyone? It'd just be neat for there to be some benefit to investing in personality, but maybe 180-200 is feasible with enchantments or something. If it's not then personality is effectively worthless, and I should make this mod. It'll never happen though if it requires modifying enemy dispositions, hopefully it can be done just by boosting effective starting personality or something. Anyway, wanted to make sure I got that down so I don't forget it again. Add it to the list of ideas, one day you'll see a thread with like 50 mod ideas I've got stored up.