Is it possible to track an iPhone which is still boxed unopened?

I have one in my possession which I'd like to sell
Its brand new and still in selophane wrapper
There's no sim card in it and I possess the delivery note which shows the imei number and also have sim card it was issued with

Track it, no. But as soon as you sell it, and its turned on and tries to connect, then it will be noticed and blocked and it won't work.

Until it is activated - no while there's no power and it is not connected to any network either by mobile data or by Wi-Fi it can't be tracked. At that stage you may as well try to track a cabbage! Unless you possess magic it is untrackable!

That does not mean that it can't be banned by IMEI though! Stores, both physical and online record the IMEI numbers of phones which pass thorugh through their inventory. When goods are stolen, they have the IMEI numbers and will usually ban those devices making them worthless!

Yes. Using your eyes.