I'v played through this game a few times and was able to sell anything to anyone who runs a shop...
Recently I started over and found these same shop owners are very particular in what they buy like farengar only buys books and necklaces when before I cood flick off anything and he'd take it.

What Happened?

you probably had the perk that let you sell anything to anyone, under the speech skill.

You need the merchant perk to sell anything to anyone, otherwise only a few will buy all categories of equipment, usually general traders such as Belethor.

Only general merchants will buy anything. Except stolen items of course. Unless, like the members above me said. You had speech perks.

Court Wizards and Collage of Winterhold : Magical items.
Blacksmiths and Archer shops : Smithing related items, weaponry and armor.
Jewelry Stands : Stones, Ingots, Jewelry.
Alchemy shops : Ingredients, potions and food.
Food stands : Ingredients and food.
General Merchants : Anything not stolen, or none-quest item.

Woops, sorry guys, I went in another direction with this new character and put nothing into speech...

It's good that you look at which perks you pick.