I haven't received my royal mail parcel?

I haven't received my royal mail parcel?
I ordered something online and it said they will send it standard royal mail.
The person contacted me who I bought the item off and said they had sent it off last Monday morning and it would take 3-5 days to come, It has not came yet.
I have been in every day and not heard any knocking or had a note through the door saying they called.
I paid a lot of money for this item as it's a iphone but why is it taking so long to come.
The person i bought it off as rang up royal mail and they said it should of been here by saturday but obviously its Monday and it still hasn't came.
Where is my parcel!
Could someone tell me what to do?
I have kept messaging the seller but we both don;t know what to do now, The seller is a good person as I have bought things from them before so please don't say its the seller, he has also shown me the recite so they have sent it off.
Someone explain?
The seller is also from the UK and only 3-4 hours away from me aswell.

Added (1). They have rang royal mail but because they didn't send it 1st class they don't have a tracking number so royal mail won't do anything about it.

Added (2). They don't have a tracking number because it was sent standard.

The responsibility is with the person sending the parcel to contact Royal Mail to find out where it is. If they can't find it then the seller has to give you the money back and take up the matter with Royal Mail.
The seller should have had some proof of posting given to them by Royal Mail.

They were rather stupid not sending it recorded first class, it is expensive but when things go wrong its much easier to deal with, it is up to the person who posted it to chase it up

Odd not to have a tracking number. I would be fearful, that this person is scamming you.