You can still (12 years after the VCRA) find antique/second-hand/militaria dealers who sell the odd airgun, blithely unaware that they are breaking the law (though probably doing no harm to public safety) and need to be RFDs to sell post-39 airguns through their business.
Then there are private individuals selling bits of their collection. Who don't need a RFD.
There is a grey area. The private individual who both buys and sells a lot, and who might be judged to be running a business.
Which category does the case you mention fall into?