Sellers say first to engage punter is to gain advantage (2,3,6)
I believe the answer is:
go one better
'to gain advantage' is the definition.
I know nothing about this answer so I can't judge whether this works.
'sellers say first to engage punter' is the wordplay.
'sellers say' becomes 'goon' (I can't justify this - if you can you should give a lot more credence to this answer).
'first to' says to take the initial letters.
'punter' becomes 'better' (someone having a bet or punt).
The first letter of 'engage' is 'e'.
'goon'+'e'+'better'='GO ONE BETTER'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for go one better that I've seen before include "Perform more impressively (than someone)" , "Outdo" , "achieve superior performance" .)