First, it would help if you describe what you are trying to calculate in column H. To do that I expect that you would also have to describe what Weight, Box, and Net, and Decrease/Increase mean.
Column H in "Buy" is the weight of the container.
Weight is the total weight of the product, including the container. First, I want to differentiate the product by it, but I'm afraid I may come into container with the same weight. So I was thinking of using the number of the bill which is unique.
Net, is the net weight of the product, so it will be 
=G2-H2
It seems to me that you can't just match on any coupon number in Buy because the Net is different for each item. Do you need to match on both Coupon and Type?
If it is possible, yes. but I think a match on the Coupon will be enough, also just in case a buyer will buy not the whole type of product. ie: the total net weight in "Buy" is 5515, the buyer only take 2000.

Also, in Sell, you have two rows with the same coupon number but different buyers. How can two buyers be on the same bill?
It's because we only use the bill for a reference. In circumstances, we open a new bill for the buyer. Sometimes the buyer only buy a type of the product, not the whole.

Regards,
Peter