Do you mean you want to calculate the standard deviation of the number of products bought? Try this "array formula"
=STDEV(IF(B2:B10>=TRANSPOSE(ROW(INDIRECT("1:"&MAX(B2:B10)))),A2:A10))
confirmed with CTRL+SHIFT+ENTER
So in a small example if you have 1 product bought by 4 people and 2 products bought by 3 people that will give you the standard deviation of the following values
1,1,1,1,2,2,2
Is that what you need?
Note that you can also use this "non array" version
=(SUMPRODUCT((A2:A10-SUMPRODUCT(A2:A10,B2:B10)/SUM(B2:B10))^2,B2:B10)/(SUM(B2:B10)-1))^(1/2)
which calculates standard deviation by calculating the square root of the average of the squared differences of the values from their average value