The word "purchase" is very similar to the word "buy."
In these sentences, "purchase" is a verb:
Our neighbors purchased a new car.
Bob is purchasing some real estate in Florida.
How many gifts do you need to purchase?
Andrea purchases everything with her credit card.
You have to purchase one more item in order to get the 25% discount.
In these sentences, "purchase" is a noun:
Vick needed to make a very large purchase.
If you make a large purchase and put it on your credit card, the credit card company will notice.
The purchase of a house is a big decision.
Purchases of food and beverages by customers who stop to buy gas help gas stations make a profit.
He’s making a purchase.
Note: The word "purchase" might be chosen over the word "buy" because the level of diction this word represents is relatively higher. It depends on the person who is speaking and the situation in which this word is used.