A string of frauds prompted a swift response from Valve.
Valve is combating games that attempt to peddle fraudulent in-game items by adding several pop-ups if players receive offers for items from games they have never played. The measure is meant to combat fraud in light of two recent games attempting to trick customers with fake rare items for Team Fortress 2 and Dota 2.
Source: N4G PC Valve Adds Pop-Up Warnings On Steam to Protect Users From Fake Item Trades