To prevent trading scams, Steam introduced their mobile authenticator: Steam Guard.
Since then, to perform a trade, each user needs to have Steam Guard installed and confirm the actions via the mobile app.
To install the app, please follow the link:
Alternatively, you can scan QR codes:
iOS
Android
The app requires a phone number to register; we highly recommend reading the mobile authenticator FAQ prior to installing.
Important: some Steam users are allowed to trade without Steam Guard enabled, however, this applies a trade hold that lasts up to 15 days. A trade hold is a precautionary measure that leaves the trade in non-terminal status for a certain period, letting a possible victim cancel the deal and get back their items.
DMarket cannot wait up to 15 days for the user’s decision, and therefore we decline all trades that are going to be put on hold.