Where many paintings, etc., are bought and sold (4)

I believe the answer is:

mart

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'where many paintings etc are bought and sold' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.

'where many paintings etc' is the wordplay.
'where many' becomes 'm' (Roman numeral for a thousand. I am not sure about the 'where' bit.).
'paintings etc' becomes 'art' (works of art).
'm'+'art'='MART'

Can you help me to learn more?

(Other definitions for mart that I've seen before include "Take tram to market" , "Place" , "Trading centre" , "US shop; market" .)

I've seen this clue in the Evening Standard.

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