This is just wrong. And it's not the first time I've seen V5s for sale on eBay.
The only reason I know for the retention of a V5 after a car has been scrapped is where you have retained sufficient components to allow the construction of another vehicle for presentation for an IVA inspection after which the new vehicle is issued with an age related registration number, not the donor car's number.
If there is an engine available that matches the details on the registration document then there probably isn't an issue. But selling a V5 is tantamount to encouraging wrongdoing. Lock up your Fulvia HF1600 and Fulvia Montecarlos.