Check my account for restrictions
There are many trade restrictions on Steam that can be applied for different reasons (for example, changing your nickname, resetting your password, etc.). You can check if your account has any restrictions by following this link.
If you see the standard trade screen, your account is fine. Otherwise, there will be an error message. You can find more information about your specific error on the relevant FAQ page.
Additional information about trading restrictions is also available on the Steam Support page.
Steam Guard
You can check whether Steam Guard is enabled and how long ago you enabled the mobile authenticator by following this link: https://store.steampowered.com/phone/manage
You can read more about Steam Guard restrictions on the FAQ page «Something is wrong with your account».
Trade holds
To check whether you can trade without delays, follow this link and add your item to a trade. If it is held, you will see the corresponding message from Steam.
You can read more about this on the FAQ page «Steam Guard is enabled via email».