SUM value by category

I don't agree that this is the right way to do this, using measures you can calculate this information display it without needing to store the value on every single line in your data set (which is what a calculated column will do).  Never the less, this is the calculated column code to do what you are asking:

This will produce your calculated column for sales

Total Sales = var thisPerson = [person]
RETURN
CALCULATE(
	SUM('YourTable'[sales])
	ALL('YourTable'),
	'YourTable'[person] = thisPerson
)

This calculates the average

Total Average = var thisPerson = [person]
RETURN
DIVIDE(
	[Total Sales],
	CALCULATE(
		COUNTROWS('YourTable')
		ALL('YourTable'),
		'YourTable'[person] = thisPerson
	)	
)