If the sale is for cash, it occurs when the goods or services are given in exchange for an immediate payment whether in term of notes, coins, cheque or credit card
If the sale is on credit, the business will issue sales invoices for the goods or services supplied showing details of the goods or services supplied, the amount of money owed by the customer and the TERMS OF PAYMENT eg: payment requested within seven days or within 30 days,etc)
Purchases for cash:
If the goods are paid for immediately namely in cash the purchase occurs when the goods and cash exchange hand
Purchases on credit:
If the goods are bought on credit, the purchase normally occurs when the business receives the goods and invoice from the supplier.