Why does eBay allow multiple listings, same used item & picture, from different sellers?

[spaulmeek]

eBay does not allow a seller to post the same item more than once. OK, so why does eBay allow the same used item, same pictures and description, to be posted by 5 or more different sellers?

[eburtonlab]

It is easy to tell when one seller lists the same item multiple times using a simple script that checks for duplicate listings.

It is not as easy to tell when different sellers are referring to the same specific item -- at least not by using a simple script.

[annadryl]

How are they going to know to begin with? There are 2.1 billion live listings right now.