Just off hand, the two weeapon dealers in Whiterun along with the general goods store, and the drunken huntsman usually hold around 3000-5000 gold total, other than that I would assume that other blacksmiths/weapon vendors in other cities. 35 hours in and I feel like I only know one city.
Pretty sure only blacksmiths/weapon shops purchase daggers. You could sell it to Thieve's guild fences too I guess. If you want to make money, I suggest enchanting jewelry instead. 1 iron or translates to 1 gold ring (if you transmute). You can then sell the jewelry right away to a court wizard. Jewelry also weighs less.
Now instead of calling idiots "idiots", we give them the benefit of the doubt and call them intentional entertainers. -(TVEW on Trolling)
I read that the thieves guild eventually unlocks a merchant with 4000 coins, and you can put perks into Speech to increase profits, have all merchants buy any items, and increase the coin they have available.
Windhelm have a enchant table right on the side of the forge and 3 merchants that buy weapons right there too, so you don't even have to walk all around the town to unload the crap you make while leveling.