cant sell my cards?

Try to do it on a browser

Same happened to me yesterday. Try from a browser, that worked for me

Browser is the answer.

I just sold a card so it's just you, are you on a new device/did you see a SteamGuard message when you logged in?

Too many cookies

Same problem here.

I'm using Mozilla Firefox and I've never been able to buy/sell on market (and I'm not in any restriction by the rules). When I messaged Steam Support, they told me to use Steam client for all market functions. I've started using Steam directly for market selling and buying but since last updates, most of the time I get message about requirement to enable cookies. I've googled a solution about editing options with cookies in Internet Explorer but it didn't help.

I'm also not in the SteamGuard restriction, I'm using Steam only on my laptop.

Same has been happening to me for a week now, so I just use the market in chrome till it's fixed.

Yeah, have the same issue. Works fine from browser tho.

Same problem. Like everyone said, just use your browser.

I had the same problems in a client for a few days. But it was working fine in a browser.

I had the same problem couple of days ago.
Restarted Steam and it worked. Interesting. :D

Try to do it thru your browser.. that's what I do since the Steam interface has been acting up lately. Even when I have access to sell directly thru Steam it ends up giving me some red error message when I finalize add a card to be sold, it says there's a problem with Community Market or my account is not allowed to sell items. But I then go to my browser and I can put the card without any hassle.. go Steam.

Today I couldn't even propose a trade using Steam. Had to do it in a browser...
Not that big of an inconvenience but still.

Found something that actually worked!

It doesn't work from steam if you're logged into browser. At least that's how I fixed it.

Same problem here, cant sell in client, only browser. Solved after restarted Windows.

im having the same problem, the only thing i can think of is i opted in for the beta client cpl days ago
EDIT restarting my client fixed the problem

whenever it doesn't let me use the marketplace I just log out and then back in and it usually works again.