tl;dr: Starting April 23rd, all sellers will need to add and verify a phone number in order to deposit any items.
Starting April 23rd, 2019 (one week from the date this blog post was published), sellers will find that they need to add and verify a phone number in order to deposit items. This is a process that takes only a few seconds, and helps increase the security of your account and of Marketplace.tf as a platform. In particular, this change helps us prevent scammers and scammer alts from liquidating stolen goods on Marketplace.
Click here to add a phone number to your account.
Put simply, we will never send you any text messages or calls that you do not want. When you verify your phone number, we will send a single SMS to verify your ownership. No further SMS messages will be sent unless you explicitly request it -- for example, if we were to offer an opt-in SMS notification when you sell an item in the future.
Additionally, your phone number will never be sold or used for advertising purposes. We do not financially gain from your phone number other than how it helps us secure our platform. Specifically, we only share your information with other entities when we are required to by law (which is not something that has ever been demanded of us by any government agency) and to prevent fraudulent payments.
While we understand that this will negatively impact some users, we have made this decision based on the fact that in order to deposit items, users need the Steam Mobile Authenticator enabled, which requires a phone number. Therefore, any users who were depositing items already have a phone number that they've attached to their Steam accounts.