If you wish to hide your activity from a contact of yours you can block them or remove them from your friends list. To block them you need to go to your Profile -> Friends and then find the contact, tap on their name, and block.

Alternatively, you can set your privacy settings to ghost mode, but this way all your friends would not see your activity, and instead, they would be notified that someone has acted anonymously.

You can do that by changing the visibility settings of action, like sales or purchase.

If you'd like to learn more on how to do that, please see this article: How to define the visibility of your trades in the activity feed

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