In most model answers for section C questions on SPL they tend to put expenses such as Cost of Sales as negative figures, but this is not how expenses are presented in real world SPLs and also not how I am used to working. I prefer to put all expenses as positive figures and then simply subtract them to calculate profit.
Yes, you could be penalised if it isn’t clear that the amounts have been deducted. I’d therefore look to follow the sign convention seen in the model answers.