I am now at level 32 in the game, and all the items that I can buy from the shops seem like too basic compared to what I find in the dungeons. Are there any special shops with very powerful items that I could have missed?
Since Daedric items can be hard to come by, it might be worth transacting with this merchant if you're at a high enough level. Since this guy can be summoned almost anywhere, it is easier to buy and sell items from him.
In my expirience (lvl 46, only been to around 4 cities and done quests) the vendor you sell most to, gets better items.
At least the smith next to my house seems to have better items than most other vendors I've found (esp. compared to the guy at the Skyforge). It could just be a coincidence, but I'd like to think that vendors you traded more with, got better and better items.
I am level 51 and the best things in game I bought in Whiterun and made them better by my character. Other good things I got through quests or made them by myself smithing and evolving magically. An ebony greatsword goes from 37 damage to 87 through smithing at about 160 mostly due to selfcreated items and then I add two magic effects. Things like these can not be bought like the Nightingale Bow. Other best things you can get from Daedra or Dragon Priests.