I was reading a Japanese text, and came across this piece of dialogue : "なんで他人とケンカするの" "なんでって...売られるからだよ.売れられたケンカは買う.そんだけだ." But I really can't seem to understand what the second person is trying to say by "売られる" ? "売る" is supposed to mean "to sell", is he trying to say that fighting sells basically ? I also can't seem to understand what the text is saying by "は買う", even though it means "to buy" technically. I just really can't understand the use of these two verbs here.
Learning phrases like 喧嘩を売る 喧嘩を買う is a hard part of any language because they are often not even thought to be idioms or set phrases. So you will not find them in a ことわざ book or a 四字熟語 book. In the case of 喧嘩, you might find example sentences in the regular dictionary entry for 喧嘩, but for any language, not only Japanese, it is good to also have a collocation reference. Here is one I have read before and found interesting:
But really any will do. It's very interesting to see which nouns and verbs go together and the different meanings you get when you combine them.
In this specific situation
means to pick a fight / throw down the gauntlet, and
means the opposite of "back down": to accept a challenge