Yesterday I updated the deposit items page to make the user confirm the bot's join date when confirming the trade on their mobile device. You can see that here https://marketplace.tf/deposit
The issue on marketplace is once a user submits a trade offer to deposit their items, the hijacker will cancel that trade offer and make a new one to a fake account that they control. When the user goes to confirm that trade on their phone they would not notice the accounts were different (since they would use the same name). Making the user confirm the join date (which is much harder to copy) increases the security here as long as the user pays attention and actually checks it.
Another case was where fake bots would send random trade offers to users taking their items and saying they would be deposited. We added a message to the main site to warn users that we will never do this and that they are completely fake.
The situation here is a little less clear to me. I think what happens is hijackers will do the same thing in canceling legitimate trade offers from us, and then send a fake one to the user. On scrap.tf this is a much harder sell since you do all trade interaction on our site. As long as the user says on our site and clicks "Open Trade Offer" on this popup they will be shown the correct one from us. Here we warn the users to only use this button, and not their steam trade offers listing since that can be fooled with fake accounts. If the hijackers cancel our trade offer; the user will be shown this message warning them if they did not cancel it to check their account with these steps: https://scrap.tf/help/kb/213
I think these are both good solutions to mitigate the problem. The solution you talked about won't work because the hijacking occurs at the trade confirmation step, and less at the trade offer step. We also want to make things as simple as possible for vulnerable users to understand, as they are the ones most fooled by these attacks. It's hard for a lot of users to understand what went wrong here since in their mind they seamlessly traded with one of our bots, and don't want to believe that it was a fake.