I think you are wrong about that. Even if all the private sellers registered as business sellers, they would still have to report the sales over the threshold amounts to HMRC, and would still require the NI number to do so. The requirement for NI numbers is ongoing, and new sellers, or those who have not sold for some time are being asked to submit their NI number before being permitted to sell.
I would like to see what evidence you have that ebay have had "a kicking" from HMRC. The requirement for reporting sales above the threshold amounts isn't just an ebay thing, it is happening with all the online marketplaces where HMRC think people are hiding substantial earnings which are not being included in tax returns.
The only genuine reason for anyone to get uptight about the reporting of sales above the thresholds is if they are indeed, hiding trading income from HMRC and guilty of tax evasion. Those who are selling their own items and are below the actual tax allowance amount will not be taxed, and if they are above the amount they would be guilty of tax evasion if they didn't declare it.
