Considering important sales becoming lousier by day’s end (6,9)

I believe the answer is:

taking seriously

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

'considering important sales' is the definition.
I can't judge whether this definition defines the answer.

'lousier by day's end' is the wordplay.
I cannot really understand how this works, but
an anagram of 'lousier' is 'eriousl' which is within the answer.
This may be the basis of the clue (or it may be nonsense).

'becoming' is the link.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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