Just a thought, but these three skills, if they're actually skills in skyrim, are pretty much forced on the pc. You'll always try to block in combat naturally and you'll also sell and buy a lot of stuff. Being that all skills start off the same and there's no class system, will everyone be forced to have these skills define their character?
First off I think I understand what your issue is. You don't want to level your character excessively by doing things that you consider to be minor, but which are necessary for you to progress through the game(like sell loot, run around and block to avoid blows). Am I right?
Okay with that said.
1) Athletics is out, so no worries there for running around. (and yes I stand firm in the belief that its gone and not 'merged' with acrobatics)
2) Block is only one form of evasion along with shield spells, dodges, and simple sidesteps or parries, so if you don't want it to define you, it doesn't have to be your 'go to' style of defense, and even if you did use it seldomly, it wouldn't play that big of a role in levelling you up.
3) Mercantile is a bit different because we all use it to some degree. But you have to remember that it can only level up as fast as you can buy and sell gear/loot/potions/etc., which means that you either have to have the gold to do excessive amounts of shopping or you have to have large quantities of loot and gear to sell. Either way, you will have to have played quite a bit with your 'defining' skills to have met one of those two demands--so don't worry about it. (and yes I know that sounded confusing but it was hard to explain)
4) You have to remember that even if you start to level quickly because you're using skills out of neccesity rather than out of desire, there will come a point where that process will begin to slow, because those 'undesirable' skills will begin to get higher themselves and be harder to level up. So once again--I wouldn't worry about it too much.