Let me back up a second. You have a list box that contains the sales codes and their descriptions. I'm assuming the Sales Codes is the bound field of the list box and is hidden (column width = 0). So all the user sees is the description for the sales codes. Correct? I'm going to assume that the Sales Code is numeric.
Now, since you want the report to print only those records that match the sales codes selected in the list box, the tag property of the list box should reflect what your SQL statement will be looking for. In your case, the SQL select statement of the report would be "SELECT [12 Month Table].[salescode], [12 Month Table].partid FROM [12 Month Table] WITH OWNERACCESS OPTION;". Note that there is no Where clause at this point.
I'm assuming that [12 Month Table] contains a field [SalesCode]. And this field contains the code that could match one of the items in the list box.
Therefore, the Tag Property of your list box should be Where=[12 Month Table].[SalesCode],Number;
Now, when you open your report, open it like this:
Docmd.OpenReport "rptName",,,BuildWhere(Me)