Crossword Genius

The salesman’s favourite words? I believe you! (3,3,4)

Ross

I believe the answer is:

ill buy that

I'm a little stuck... Click here to teach me more about this clue!

'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...

Can you help me to learn more?

I've seen this clue in the Evening Standard.
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