If you want to sharpen your Dwemer war axe, you have to sharpen it, yourself.
If you want to use powerful poisons and potions, you have to make them, yourself.
Alchemy
Alchemy, it seems, can get away with it. For one thing, alchemists do sell potions and poisons. Also, I tend not to use them much anyway and just go with whatever I find lying around. Which is another problem, but we won't get into that.
Smithing
Smithing is problematic. If you are playing a mage character, there's just no possible way to get good weapons or equipment other than to make it and refine it yourself. I don't agree with that. I should be able to take my kit to a skilled blacksmith and pay them some (huge) sum of money to make me a set of X. Or pay them some small fee to sharpen my blade. Or get fitted for armor. Or whatever.
Wait! How will the blacksmith of potentially unlimited skill know how much to sharpen (temper) the item? Well, you can have several things involved in that. First, your skill with the weapon/armor in question could come into play. Light armor skill of 75 means you can buy a better fit than skill of 20. Also, you can have the player level come into account. I'm less a fan of this option, but maybe. You also might just have certain blacksmith's have different skills and levels. Eorlund and his Skyforge, for instance, might be the only one who can do certain things.
(You could really expand on this, too, considering that there is the Lunar Forge as well, and previous games have Dwemer Forges. You could use unique forges in quests to unlock certain things that you could buy from certain blacksmiths.)
Lastly, you can just use money (and materials) as a barrier. For instance, smiths can't make you a full set of tempered daedric unless you bring them at least as much (perhaps more?) material as you would have needed in order to do it yourself. Plus you would need to pay them some huge sum, like 45,000 or something. I don't know exactly, but it's certainly possible to balance.
Enchanting
Enchanting is another one. As a warrior, why is there no possible way for me to get my weapon enchanted except to do it myself? That's purly nonsensical, especially in light of the following facts:
- Sergius Turrianus often says that enchanting services are one of the few things that keep the college in touch with the rest of Skyrim.
- Farengar Secret-Fire offers to give you basic lessons on how to enchant things. (implying he knows how to do it)
- There are skill trainers.
- Guards often ask you to enchant their equipment (if you are skilled). Which you can't. Sad face. "Sure guard, I'd love to!"