Unless the seller is licensed by the DoT to sell number plates they are selling illegally.
If they are the ones I have seen with the 'plate generator' in the listing, these are not legal and are sold as 'showplates'.
Any legal seller of number plates will need your proof of id and proof of entitlement to that registration number. I don't think I would be happy giving that information to an eBay seller - although I don't know why!
Regarding background pictures - these are certainly illegal.
The front must be plain white, the rear plain yellow.
The only emblems allowed on number plates are the Union Flag, the national flags of England, Scotland or Wales, or the EU stars GB flag.