Allen Xiao, Native speaker of Chinese,always trying to learn a bit more about my language
Answered Mar 17, 2016
There's a pretty interesting(pseudo)historical anecdote that seeks to explain why- in fact, it's so obscurethat I had to ask my uncle, a major in Chinese literature, to confirm it!
The story goes that emperor Qianlong wasvisiting a market, and asked an old man: 'why does your store sell 東西-east/west- and not 南北- south/north?'
To which the shopkeeper responded:'North iswater, south is fire. East is wood and west is metal. And my basket can onlyhold the last two things. So I could only sell what's from east and west, andnothing else!' (Note that in the traditional conception of Feng shui, thedirections really were associated with the aforementioned elements)
Amazed, Qianlong paid his respects to thiswise man and apologised for his arrogance.

這里面有一個(gè)非常有意思的歷史故事，可以解釋這個(gè)問題，但是我不很清楚，所以詢問了中國(guó)文學(xué)專業(yè)的叔叔。
故事是這樣的，有一天乾隆來(lái)到一個(gè)市場(chǎng)里，問一個(gè)老人：為什么你商店里賣東西（東和西）而不賣南北（南和北）？這名店主回答道：北是水，南是火。東是木，西是金。我的籃子只能裝下后面兩樣?xùn)|西。所以我只能賣來(lái)自東邊和西邊的東西。在傳統(tǒng)的風(fēng)水概念里，每一個(gè)方向都和上面所提到的事物相關(guān)聯(lián)）
乾隆感到驚訝，向這位智者表達(dá)了敬意，并為自己的無(wú)知道歉。

What's more interesting is that thehistorical era for this anecdote vary- other sources give it as Zhu Xi (afamous Ming Dynasty scholar) visiting a friend. However, the bulk of the storyis still the same, indicating a rather popular folk etymology for the word,which has held itself throughout hundreds of years of transmission.
As colourful as it is, there's probably amore banal explanation. Around the 15th century, China really began tradingwith the world, and it so happened that most of her trade goods came from theeastern and western oceans (ie Japan and Arabia). Markets sold 'wares from theeast and the west', becoming simplified to 'east-west wares' and eventually'east-west'. Alternatively, it was just a simplification of 東西南北中 (thefour directions plus the centre), which once again would indicate that one'swares came from all directions. Compare this to 春秋, which couldeither mean a specific period in Chinese history or 'the ages/seasons of life'in general- this being a truncation of 春夏秋冬.
東西 itself seems to be a recent invention anyway. In Classical Chineseit was 物事, or 'Objects and affairs', and it was only until the Ming Dynastythat 東西 was used as a compound/substitute in relation to 物事.

更有意思的是這個(gè)故事到底發(fā)生在哪里歷史時(shí)期有著不同的說(shuō)法。有的說(shuō)是朱熹訪問朋友時(shí)發(fā)生的。故事內(nèi)容差不多是一樣。
雖然豐富多樣，但是還是有一個(gè)比較平凡的解釋。大約15世紀(jì)左右，中國(guó)開始和世界貿(mào)易，大部分的貿(mào)易商品來(lái)自東洋和西洋（即日本和阿拉伯）。市場(chǎng)里銷售的物品來(lái)自東洋和西洋，所以就簡(jiǎn)化成“東西物品”，再后來(lái)簡(jiǎn)化成“東西”。或者說(shuō)，它是東西南北中的簡(jiǎn)化，指的是某人的物品來(lái)自各個(gè)方向。比如“春秋”可以指中國(guó)歷史上的某個(gè)時(shí)期，或者指生命里的年齡——截取自春夏秋冬。
東西本身貌似是近來(lái)的發(fā)明。在古漢語(yǔ)里，叫物事，直到明朝東西才用來(lái)替代物事。

Hong Zhang, edit
Answered Mar 15, 2016
One story is that 'dōng xi' datesfrom Tang Dynasty, whose capital was located in 'cháng ān', as yousee in the picture below.
Chang'an Imperial City was located in thenorth, surrounded by many oblong 'fāng, lanes' and 'shì,market' enclosed by high walls. In the east and west of Chang'an City,there were two markets, namely dōng shì, East Market and xī shì, West Market.They were both a large goods distributing centre. At that time, residents wereforbidden to buy goods along the roadside, so they had to go to the East Marketor West Market instead. Shopping at the East Market was called 'mǎi dōng',and 'mǎi xī' at the West Market. As time passes, 'dōng xi'has become a word meaning goods and then the phrase 'mǎi dōng xī' hasspread far and wide.

有一個(gè)故事說(shuō)的是東西源于唐朝，當(dāng)時(shí)首都是長(zhǎng)安。長(zhǎng)安皇城位于北邊，被很多長(zhǎng)方形的坊和市圍繞著，外面還有高墻包圍著。在長(zhǎng)安城的西邊和東邊，有兩個(gè)大市場(chǎng)，即東市和西市。二者都是大型的商品分配中心。當(dāng)時(shí)，居民禁止在路邊買東西，所以必須去東市或者西市。在東市買東西叫做“買冬”，在西市購(gòu)物叫做“買西”。所以東西就演變成了商品的代名詞，然后“買東西”這個(gè)詞就得到了廣泛的傳播。