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Every one, except the riches, knows shark fin soup is expensive and luxurious, it is a known fact.

Using "seem to be" is not adequate, "seem" means "appear to one's own opinion" 被認為

People kill sharks cruelly in order to get their fins, this is another known fact.

Similarly, using "turn out" is not suitable, "turn out" means "be found to be".被發現

Please justify.

Sentence rewriting will be left to others.

I would write this sentence in the following way:
Shark fin soup (seems) to be a (luxurious) dish, but it (turns) our to be a food (made) (in a ) cruel way.

Hope it sounds reasonable!

Shark fin soup seems to be a luxury dish, but in fact it is a dish made in a cruel way.