This is a controversial topic. The answer of ttspeakseason makes sense. I once thumbed up it, but now it said that I was not allowed to do it twice. So I put my answer here. Despite this, some ones may argue that the explanation above is not very reliable, because they didn't happen at the same time that "东西" being seen in books and "east west market" being established. Another explanation is that it is a symbolisation with distinctive Chinese flavor. For example, because things happen from spring to winter, so we say "spring winter"（春秋） instead of "history". Similarly, because things grow in the east, west, north, south, so we say "east west" instead of "stuff". I assume that maybe because conventionally, we only pick up the first one or two words instead of the whole. (The normal sequence in Chinese is: east-west-south-north-center) While streatman's opinion is: Chinese ancestors viewed “the five elements” (mental, wood, water, fire, earth) were corresponded to east, west, south, north, center, respectively. While in these five elements, only mental and wood can be taken as containers. Hence people begun to use "east-west" to imply "things". While I think this explanation is probable made up by modern people. Because to some extent, "mental" and "wood" are only an abstract and general concept, they don't not really refer to mental and wood physically.

In ancient times, only west and east have stores in city, so if you want to buy and sell items, going to weat or east stores, so becoming 東西 which means items/things.

东西南北中 也代表着五行 东和西分别代表金和木，古代宋朝，一般上街买物品就是买金或者木，才有东西一说 南和北是水和火，表示不相融合，所以采用买东西而不买南北

东西这个词跟宋朝有关 学中文不用刻意追求某个词的来历，要不然，你会晕的 明白意思就好····

宋朝的理学家朱熹好学多问，有次他在巷子里碰到精通五行的好友盛温和，问道：“你提着篮子去干什么啊？”盛温和幽默的回答：“去街门买东西”。当时还没有“东西”一说法。朱熹不解地问：“买‘东西’？这是什么意思？为何不买‘南北’？”盛温和并没有直接解答，他笑着回敬说：“真不明白？你这位大学问家真是聪明一世糊涂一时啊。你把五行和五方对照一下就会豁然开朗了。”朱熹独自思付，“东”即“木”，“西”为“金”，“南”属“火”，“北”乃“水”……突然他如梦初醒，原来盛温和是用了诙谐的语言说，“上街去买金木之类，可装入篮子的物品”！若说“南北”当然不对了，篮子里怎么可以装水和火呢？从此，“东西”就成为了物品的代名词直至今天。 　　当然这个只是古人的迷信之说，可做参考。 　　不过，“买东西”一词,自古就有。在东汉时朝，洛阳和长安是两座商业繁华的城市，洛阳被称为“东京”，长安被称为“西京”民间有人到东京、西京购货，及称“买东”“买西”。久而久之，“东西”便成了货物的代名词，于是买“东西”一词就流传开来了！