How are 買 and 売 related?


It has always struck me as strange that 買 "buy" and 売 "sell", which are
antonyms, have the same _on_ -reading バイ. A few questions:

  1. Is there some logic (possibly stemming from the Chinese use of these characters) behind the readings being so similar? 
  2. How are the characters 買 and 売 related? Is one derived from the other?

(This post is inspired by the comments on [this
answer](https://japanese.stackexchange.com/a/17498/3437), which ought to find
their way into a proper answer rather than languishing in the comments
section.)




### The Chinese part

The answer to this question ultimately goes back into the Chinese from whence
these characters originally came.

> ​1. Is there some logic (possibly stemming from the Chinese use of these
> characters) behind the readings being so similar?

The _on'yomi_ come from Chinese. Both 売 (selling) and 買 (buying) are simply
two sides of the same activity, so it kind of makes sense that they're almost
the same word: 売 (in Mandarin, _mài_ with a high falling tone), 買 ( _mǎi_ with
a low tone rising slightly at the end).

> ​2. How are the characters 買 and 売 related? Is one derived from the other?

Courtesy snailboat: Yes, one is derived from the other. The modern Japanese
kanji 売 is a 新字体{しんじたい}, a simplified form. The old character is 賣, which is 士
over 買. The 士 element here is itself simplified from older 出 " _to put out_ ",
so 売(賣) = 出＋買 = " _to put out for buying_ " = " _to sell_ ".

### The Japanese part

This isn't part of the original question, but in the interests of
completeness, we should also look at the etymologies of the Japanese words
_kau_ "to buy" and _uru_ "to sell".

  * Japanese 買{か}う is, at its root, the same word as 代う, 換う, 替う, 交う, all かう with a root meaning of "to trade, to swap, to exchange". By extension, these are the roots of かわる・かえる "to change, to turn [into]", and probably also かえる・かえす "to turn back, to return". See also [the Gogen Allguide entry for 買う](http://gogen-allguide.com/ka/kau.html). 

かう derives from older かふ. This seems to be related to 食{く}う (older くふ) "to
eat, to put into one's mouth" and 請{こ}う・乞{こ}う (older こふ) "to ask [for], to
request". All three are semantically related to ideas of _getting_ or _taking
in_.

Much more speculatively, these _inward_ semantics make me wonder if these
might be cognate with 来{く} (modern 来{く}る), with the final ふ perhaps the well-
known auxiliary that attaches to the _mizenkei_ and indicates repeated or
continuous action. If so, かふ would meet the expected form of the _mizenkei_
(with an _a_ sound), but くふ and こふ would not, leaving significant holes in
this theory.

  * Japanese 売{う}る, meanwhile, is cognate with 得{え}る (also read as うる) "to get, to acquire", from the idea of _getting_ money (or something else desired) in exchange for the item sold. See also [the Gogen Allguide entry for 売る](http://gogen-allguide.com/u/uru.html).




### The Chinese part

The answer to this question ultimately goes back into the Chinese from whence
these characters originally came.

> ​1. Is there some logic (possibly stemming from the Chinese use of these
> characters) behind the readings being so similar?

The _on'yomi_ come from Chinese. Both 売 (selling) and 買 (buying) are simply
two sides of the same activity, so it kind of makes sense that they're almost
the same word: 売 (in Mandarin, _mài_ with a high falling tone), 買 ( _mǎi_ with
a low tone rising slightly at the end).

> ​2. How are the characters 買 and 売 related? Is one derived from the other?

Courtesy snailboat: Yes, one is derived from the other. The modern Japanese
kanji 売 is a 新字体{しんじたい}, a simplified form. The old character is 賣, which is 士
over 買. The 士 element here is itself simplified from older 出 " _to put out_ ",
so 売(賣) = 出＋買 = " _to put out for buying_ " = " _to sell_ ".

### The Japanese part

This isn't part of the original question, but in the interests of
completeness, we should also look at the etymologies of the Japanese words
_kau_ "to buy" and _uru_ "to sell".

  * Japanese 買{か}う is, at its root, the same word as 代う, 換う, 替う, 交う, all かう with a root meaning of "to trade, to swap, to exchange". By extension, these are the roots of かわる・かえる "to change, to turn [into]", and probably also かえる・かえす "to turn back, to return". See also [the Gogen Allguide entry for 買う](http://gogen-allguide.com/ka/kau.html). 

かう derives from older かふ. This seems to be related to 食{く}う (older くふ) "to
eat, to put into one's mouth" and 請{こ}う・乞{こ}う (older こふ) "to ask [for], to
request". All three are semantically related to ideas of _getting_ or _taking
in_.

Much more speculatively, these _inward_ semantics make me wonder if these
might be cognate with 来{く} (modern 来{く}る), with the final ふ perhaps the well-
known auxiliary that attaches to the _mizenkei_ and indicates repeated or
continuous action. If so, かふ would meet the expected form of the _mizenkei_
(with an _a_ sound), but くふ and こふ would not, leaving significant holes in
this theory.

  * Japanese 売{う}る, meanwhile, is cognate with 得{え}る (also read as うる) "to get, to acquire", from the idea of _getting_ money (or something else desired) in exchange for the item sold. See also [the Gogen Allguide entry for 売る](http://gogen-allguide.com/u/uru.html).

