Tableau table calculation (Possible if statement)

I am looking to create a table calculation based on the following:

Data in data source:

1. Sales
2. Third Party Sales
3. Item Number

Data needed: (Calculation in Excel)

1. If([sales] > 0 then [third party sales]) --- This would be on the item level. Essentially I am looking to calculate third party sales on only items that currently have sales.

I am looking for a simple table in Tableau that would have 3 columns; "Sales", "Third Party Sales", and "Third Party Sales on items with Sales".

Please let me know what the best way is to approach this and thanks for the help.

Best, CafeRacer