Buy is used before these nouns:
signal

Buy is used after these nouns:
bargain, impulse

cheaply
Old bicycles can be bought quite cheaply.

online
Young people are very comfortable buying online.

locally
Was the produce imported or bought locally?

can afford to
We can afford to buy enough paint to do the whole house.

can't afford to
I can't afford to buy a new car.

at
CDs can now be bought at the supermarket.

for
He bought a car for his daughter.
I bought it for $25.

from
I bought some books from a friend.

buy and sell
She makes her living buying and selling antiques.

money can buy, money can't buy
It's the best that money can buy.
There are some things money can't buy.

Buy is used with these nouns as the subject:
consumer, investor, money, museum

Buy is used with these nouns as the object:
apartment, art, asset, beer, bond, business, company, condom, condominium, copy, cottage, coverage, cup, currency, drink, electricity, equipment, estate, excuse, firm, flat, food, franchise, gift, goods, grocery, happiness, home, house, insurance, journal, land, lease, licence, loyalty, lunch, magazine, mansion, membership, merchandise, mortgage, newspaper, outfit, painting, paper, pension, permit, pint, postcard, present, produce, product, property, provision, record, right, round, security, share, single, site, slave, software, stake, stock, subscription, supply, ticket, trailer, wardrobe, warranty