brought a new iPhone X from an apple authorized reseller. But this phone is demo version. Seller said demo phone is same as a brand new original phone. Only thing is not included a headset is this phone suitable to use?

Apple no longer includes the charge cube, the headphones, or the headphone adapter with any iPhone purchase. However, if it is indeed a demo device, return it for a refund if possible.

I would have thought a 'demo phone' would be just the case and a printed display for putting in shop windows - so that it doesn't matter if it gets stolen. Does yours actually work?