I used “Steam Economy Enhancer” to automatically sell the 400 random old items in my Steam account. They sold for US$25, real dollars now in my Steam wallet.
It works via a browser script. You need Tampermonkey to run the script. I know the name sounds dodgy, but it is a common tool with 10 million downloads. https://www.tampermonkey.net/
Then install this script: GitHub - Nuklon/Steam-Economy-Enhancer: Enhances the Steam Inventory and Steam Market.
The script adds new functions to your browser to list all items in your Steam inventory for sale. You need to log into your Steam account via the browser. After you do, browse to your inventory and you will see this:
Click “Sell All Items” and it will auto-list them all (hundreds and hundreds, if you have that many) by checking the current market price for the item and then listing it for that value. This takes a little while. If you have two-factor or other Steam security measures turned on, you may need to check the Steam app on your device and then check them via “Confirmations”.
You can confirm it worked properly by checking the Steam Marketplace, also via your browser. You will now see this:
You can tell it worked if you see a lot of things listed for sale. :-) If not, check your Confirmations on the Steam app on your phone. If not, check to make sure Tampermonkey is working. Or ask here.
It takes a while for all of these sell orders to turn into sales. You should notice the sale confirmations flowing in via email within a day. They will trickle in over time. If they slow down, go back into the browser Market and “Select All” then “Remove Selected” and then re-list then. That’s because maybe the market prices changed and your listed prices are not good. (Or use “Relist Overpriced” button.)
It took me a few clicks to set up. I had to click a few Confirmations (which is easy via the phone). I had to re-list old items two times (over two months). That was maybe 15 minutes of work, total. But now I have only 50 items left and my Steam wallet has +$25.
I assume you people also have a lot of items in your Steam inventory. :-)
PS: Apologies if this sounds like some horrible scam, it is not. Tampermonkey is a well known tool, and this script is open-source and widely used. Here is another public reference to this script.