Brett, I see the problem in using COUNTIF function. Try it separately, apart from the rest of the formula. It returns the number of duplicates of the value in the current row. So you need to copy it down to see this nimber of duplicates for each value. And you want your formula to find the maximum of these and then return the name.
You may also enter MAX(COUNTIF(sales,sales)) as an array formula and get the maximum number of duplicates, but if you try to use MATCH it won't work. So I suggest that you add a field with COUNTIF results, and then use your INDEX+MATCH formula search the maximum value and return the name.
Please try and say if this helps