I would also like to know. I have a bunch I'm not using.
Same
Not been available for a while now 
Even when I get them I can't sell them.
Strange, we can't seel runes but every time i get soul gems from a daily chest there is an option to sell them O_o
Really?
maybe i dont
u cant sell them
i need gems or codes
Bad thing a out codes are that you can't use them 5 days of using one
there are rumours that you could only sell them if you got the rune from a chest that you buy
not from the daily things neither those which you get free (only those which you use gems to buy)
You can sell it even if you accepted to keep it in you inventory.
Just open your inventory (chest icon), click on the rune you want to sell, and on the lower part you will see a ‘Sell +1’ button.
Look at this image, marked on red.
https://i.postimg.cc/rs6LTGNh/sell-rune2.jpg
they worth 1 if u get them from daily chests , but for exmple 75 SG on a chest u buy !
wow.. my rune only sells for 1 sg!
in 3 steps
Choose the game that interests you and play itfor free.
Every game has some quests for which you will receive Soul Gems.
You can exchange your Soul Gems for Steam Wallet top-ups, game keys, CS:GO Skins and other rewards.
You know that on Gamehag you can get real rewards for free, right? Join us, complete walkthrough and get your chest!
^ Is it only available after getting it (through chest)?