The salesman’s favourite words? I believe you! (3,3,4)
I believe the answer is:
ill buy that
Here is my best explanation:
'the salesman's favourite words?' is the definition.
I don't know anything about this answer so I cannot judge whether this works.
'i believe you' is the wordplay.
I cannot really see how this works, but
'i' could be 'a' and 'a' is present in the answer.
'believe' could be 'buy' (synonyms) and 'buy' is present in the answer.
This explanation may well be incorrect...