You probably need to have a linking element between the two tables, that identifies the item you are buying and selling - otherwise you will need two tables for each item in your store, which is silly!
Buy - it's pretty trivial to get SQL to return the value from your tables:
SELECT (SELECT SUM(saleAmount) FROM sale) - (SELECT SUM(purchaseAmount) FROM purchase)
It's just that it is useless in the real world!