Change sign of number based on validation drop down in another column

This spreadsheet will be tracking sales and purchases. I want to be able to have any purchases formatted as an expenditure (negative sign or brackets) to show it is money out. Column C "Action" has a drop down with either Buy (infrequent) or Sell. I want to be able to have column I "Unit Price" be a negative number if the corresponding cell in the same row in Column C = "Buy". I do not have these prices formatted as currency as their are 2 different currency used and I don't know how to make the correct currency format appear automatically, though I do have a drop down validation column for which currency the transaction was in, so maybe that is possible. I know this is probably simple, but I don't spend that much time using Excel. I have some sample data in their for the moment.