In November, Valve introduced a system of trade holds to limit miscreants' ability to access others' Steam accounts and trade skins away. The three-day hold was intended to let the rightful owner realise something was up and step in to stop it, because hat theft isn't a sporadic problem—Valve reports that over 77,000 accounts get cleaned out each month. Now Gabe Corp. is strengthening protection by extending trade holds and ending its practice of replacing lost items.
The duration of trade holds has been increased to 15 days, a two-week window in which to kill unauthorised trades. As under the previous system, if you use Steam Guard two-step mobile authentication—and Valve says 95% of daily trades do—there is no waiting period. Valve really wants you to use Steam Guard. If you don't or can't, trades with players you've been friends with for over a year retain their one-day hold.
Trade holds will now affect sales on the Steam Community Market too, stalling malicious liquidation of inventories. With this move, Valve considers players protected enough to end its longtime practice of replacing lost skins, which duplicated items and reduced their value. Again: if you are a Counter-Strike tycoon, enable Steam Guard to prevent the unthinkable.
It's with an apologetic wince that Valve acknowledges the changes are inconvenient for people who can't use Steam Guard. You can read the full explanation here, but the TL;DR is that the end justifies the means.