There must have been a lot of sales. My nearest antique shop (must have got them at auction) has at least 400 piled up desparate for someone with time to go through, they are not ALL Jimmy Young Decca ones), I got a few from a huge selection in a Charity Shop in Redruth 32 years ago picking some I wanted mainly as replacements for ones I had broken. Basically take the number of towns in the UK, assume three record shops per town and assume 10 sales per disc per shop and you wont be far off an average sale of any particular recording as a base line figure.