To get a close attention or vigilance to minimize the risk,
And you dont want to face those problems,
You should:

[Case 1]

I think I just lost items in a trade! What do I do?

Steam Support will not return any items or gifts that you feel have been traded unfairly. There are no exceptions to this policy.

To ensure that a scammer is appropriately handled, and to prevent them from benefitting from this scam or others in the future, make sure that you report them through the Steam Community:

Reporting a scammer through the Steam Community will always be better than submitting a Steam Support ticket containing your report. A community report includes the best information about the interaction between the accounts, and is quicker to review and act on.

[Sol]
You cannot get back your items in trade.You should revise the trade at least once.Checking http://backpack.tf is a good choice (For TF2 items trade).When you hit the button "Confirm" ,you will not yet start trading with the user (who currently trading with you).Because you have two factor authenication which steam sends you a email about confim the trade.When you hit the button,which means you agree to the trade.And sadly nobody can help return those items to you.