@HRSexton – OK, I think I see it now. Assuming that “inventory” column is the end of the day (if it were beginning, you would need another column to fill in ending?
12/1/x1 they had no existing inventory and make no sales, since the perpetual inventory matched the purchase – so Sales is all zeros
12/6/x1 they purchased 100, but inventory remained at 400@11, so they must have sold all 100 of the new items
?future date, maybe end of month/year, they now have 100 @ 12 so they must have purchased another 100 @ 12 and sold none to get an ending inventory of 100 @ 12.
Does that approximate the answer?