can i trust on refurbished iPhone 6 Plus which i want to buy from saudi souq.com? I am living in jeddah KSA.

Not for us to say. We don't really know. You may need to do some investigation re: below.

It requires the Apple ID and password of the previous owner. If you don't have it and cannot get it, then the device is useless to you. Return it for a refund, if you can, or take it to an Apple store for an exchange provided they will help you.

Find My iPhone Activation Lock

In the future check first before buying second hand: How to check for Activation Lock before buying a used iPhone.