After reading the most recent page and a half i have never seen this ticket being sold on the day before - the catch of this ticket is the fact that you had to wait a few days before using it. I reckon once the London Zones 1-6 travelcard has gone it would be nice if Southern had it as a buy on the day option on the ticket machines as its a great ticket!
When it comes to the ticket being recognized by Southern members of Staff, very few of them are aware of its existence - most times i get checked for the ticket i end up explaining to them what the Daysave does. However, after its explanation most ticket collectors really like the idea of what this ticket can do, and its a real shame that the Southern Daysave and other rover tickets are poorly advertised.