If we all want a sealed gun in its box, how can you compare two different guns? Once you've had a look at it your claiming its not new.

Regarding vat on secondhand goods. You either pay vat on the whole item or pay vat on the difference you paid for it and what you sell it for.
Cars is not so much of a problem.
Buy a second hand gun and a new tin of pellets, that two different ways of accounting vat. Also two accountants fees.

It can be easier and cheaper in the long run to charge vat on the whole item and make a smaller profit.