買う and 売る: buyer and seller
Read a transaction from the buyer's and seller's perspectives.
The buyer
買う (かう) means buying something for payment. 「店で本を買いました」 says that the speaker bought a book at a shop. で marks the place and を the item; 買いたい instead expresses a wish, not completion.
「パンを買いに行きます」 means going to buy bread. Buying is the purpose of the trip, but successful purchase is not yet established.
The seller
売る (うる) means selling something to another party. 「友達に自転車を売りました」 says that the speaker sold a bicycle to a friend. に marks the recipient; do not reverse it into buying from the friend.
「売りたくありません」 means not wanting to sell. It expresses the speaker's preference, not a market situation in which the item fails to sell.
Available for sale versus sold
「この店では切手を売っています」 says that the shop sells stamps. It does not mean all the stamps have already been sold. Check whether the context concerns availability or a completed transaction.
「昨日買った本を売りました」 means selling the book bought yesterday. Buying came first and selling later; one person can occupy both roles at different times.
Who is the buyer in 「兄から車を買いました」?
Show the answer and why
The omitted speaker is the buyer, and the older brother is the seller. から marks who the car was bought from.