example Pong buy shoes for sell. Pong is both buyer and seller.
If Pong buy shoes by 300 baht,Pong is a buyer .
Price of shoes that Pong buy is called the cost price.
If Pong sell them for student by 450 baht,Pong is a seller and student is a buyer.
Price of shoes that Pong sell is called the sale price.
When the sales price is greater than the cost price.
The difference between the sale price and the cost price calledth profit.