Market Mass Selling has been one of the most requested feature for the extension since its release. It's also what kept me from removing Steam Inventory Helper completely (I enabled it sometimes when I wanted to list many items). I am glad to announce that the feature is now in Beta. Switching currencies back and forth does not seem something that Steam takes lightly so I was not able to test the feature with currencies other than Euros. This is mostly the reason that it's still in Beta. While using please double check the prices when you are confirming them on your phone. If you find any bugs or strange behavior please describe it in a mail to email@example.com.
Click the hand icon in your inventory to start selecting items that you want to sell. Holding down the control key while selecting an item will also select all similar items. Once you have the items you want to sell in the list specify a price for it. You have four options:
Click "Start Mass Listing" to initiate the selling process
Note: You might have your extension's price set to a different currency than your Steam Wallet currency. If that is the case you will receive this warning message. Clicking "Click here to fix this" will change your extension price to the same as your Steam Wallet price and reload the page.
You will be able to see the progress by the quantities decreasing, lines getting stricken through and items becoming greyed out.
I have worked quite a bit on this and I think it ended up pretty nice and useful so I want as many people to use it as possible. Unfortunately to do so you have to add your Steam API key in the options first. I tried solving this without that but it's not really possible to do so while offering appropriate user experience. If you don't have your API key set then all you will notice is a note on top of the trade offers page advising to add your API key and linking to this page.
These are the features that you are missing if you don't have it set: