Any way to mass sell all my steam trading cards?

No, not through the steam market anyway. With the market you have to sell them all individually.

You might be able to trade them in bulk for a couple of items (like CS:GO skins) that you can then put on the market quicker.

There are actually bots in steam that buys trading cards.

One good example: http://steamcommunity.com/id/CardExchange

The buying rates may differ from one bot to another. You can search through steam for more examples.

If you are looking to sell for steam wallet (steam market transaction), unfortunately that is not possible.

Having just looked into this same challenge, Steam Inventory Helper (the originally-linked-to answer) has been reported for spying on its users. The replacement recommended in the linked Reddit discussion is Steam Economy Enhancer userscript. N.B. It must be installed through Tampermonkey (or Violentmonkey), but is open source and thus far, well received in the community.

It also works well on Firefox. Source: Personal experience of unloading all of my Steam cards in the last five minutes...

The most simple way I know is using a Google Chrome extension: Steam Inventory Helper

Some users reported it is not privacy safe, but you can just install it 1-time for selling it and uninstall it after you're done. Just don't leave it installed or visit other pages while having the extension installed.

1. Install the Chrome Extension
2. Go to https://steamcommunity.com/my/inventory/
3. Login to your Steam account
4. Press new green Button "Select items"
5. Select all Items you want to sell
6. On the right side click "Sell X items"
7. When the popup open on the left top side there is a button "quick sell all" - this will fetch the current price for each of your items and set is as the price.
8. Now the last step: You have to go to your steam app and approve all items.