You can't cancel the sale, only the seller can do this by sending you a cancellation request and only if they deem your request to cancel as valid..
A seller not replying in 24 hours is not a valid reason.. You're being impatient.. You should allow at least 48 hours..
Just because the seller has stated working hours, does not mean that they will be spending those in front of the PC. They could be packing stock, preparing orders for sale etc
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You keep using that word, I do not think it means what you think it means