OK so some of my salvaged black mage robes are classified as stolen (don't know why though, I legitimately slew their owners) however I cannot find buyers even for the non stolen ones... Not a thief guild member, tried the khajit tried the college of mages, that dodgy merchant in Windhelm. Nada!

Any help would be welcome.

Clothing vendors should buy robes. Radiant Raiment in Solitude is the only one in the game, that I know of. The staff at the mages college may or may not buy them, depending on the school of magic they specialize in and the kind of robe.

Best bet for selling robes (and potions, and other common, high value items that pile up in your stash) is to just get the "merchant" perk. Then you can sell anything to any vendor.

You didn't know you have to wash the blood out of them first, then repair them? Who would want to buy a blood stained robe with a few slices in it? Just kidding. Haven't had that problem. Bersi buys them.

Try dropping the stolen ones and see if they will buy the non-stolen ones, sometimes they get stacked together and all are classed as stolen. Happend to me with arrows, stole some and then all my arrows were marked as stolen.

Gee thanks, does anyone know of an Ariel/Persil sales rep in Skyrim?

No, seriously the robe girls in Solitude won't touch me but the last suggestion is certainly interesting.

How many do you have?

If they're not too many I'd just drop them in the river somewhere out of sight of the guards... they don't like littering or lollygagging. I know at the beginning of the game 50 gold each is a lot of money. Hmmm.... do you have a follower? Like maybe Faerndl or Sven (riverwood)? Take the key to their house out of their inventory. Go into their house and put the robes in a chest or something.

Then fight another hostile mage who is wearing one of those robes. If you remove that robe from the corpse and it says "Stolen" your game is bugged. That's all I can think of.

Did you by any chance let one of your followers pick them up...
After 1.2 items picked up by followers (of the ground or from containers) can become flagged as stolen. Combine this with the fact that all items stacked (as stated) will receive this flag and voila... Frustration ensues.
If you really want to sell them... you could start the TG (after phase 1, you will have a fence)

Two things:

1. I don't know how to order my companions to pick up anything.

2. I micromanage stuff too much to allow my companions to pick up anything.

But this is my issue, not the OP's. Then it might not hurt for me to RTFM.

Only a few people buy robes. The ones that say they buy ANYTHING will. Also the jarls wizards might buy them.

Other then that getting the merchant perk not only lets you sell anything but also BUY anything a merchant has...

Court mages will buy robes, along with enchanted and unenchanted jewelry items.

Belathor (Whiterun), Bersi (Riften), Grelka (Riften) and the court mages are the three I know of that do. Lord knows I've got enough of them.

Niranye in Windhelm does.

Revyn Sadri in Windhelm, and I believe Aval Atheron buys them as well. His stall was a hidden secret for a long time since he only displays meat, but he actually buys and sells everything and has a good pile of gold. All the court mages and Falion in Morthal, the vendors at the college, the general goods store in Winterhold.