Starting Dec 9, Steam's escrow system will go live.
It means that if you don't have:
-A linked mobile number to your account
-Steam Guard Mobile Authenticator enabled for 7 days
-Enabled trade confirmations
All your offers where you are the one sending items (deposits), will be held for 3 days.
At a point, we may decide to decline these deposits, if it turns out to be complicated.
Details:
https://support.steampowered.com/kb_article.php?ref=8078-TPHC-6195