This article by a noted biblical scholar emerged (somewhat initially) at the San Diego SBL meetings in 2007. I remember listening to the presentation and thinking that Koester was really on track here, especially with some familiarity with his overall reading of Revelation. It also came on the heels of an absolutely horrible reading of Revelation (I don't even remember the specific topic), which also helped me consider Koester's position more favorably.
What Koester gets right in this article begins with his fundamental assertion: ". . . Revelation's visionary rhetoric poses a challenge to see the world differently and to resist practices that are inconsistent with the faith" (766). This fits strongly with the notion that with Revelation we are reading biblical apocalyptic, which must be read in light of both ends of the category - biblical in that it has a message which challenges the world with God's (and Christ's) sovereignty; apocalyptic in that it is a message of hope and inspiration for the people of God.
Also consistent with a proper reading of Revelation is the distinction made between the people of God and the people of the world, which Koester gets right. There are those in the narrative who are marked with the seal of God on their foreheads (7:3) and those who are marked as the people of the earth (14:1). This is a vital difference in Revelation (and most apocalyptic): to see the polarizing positions of those dedicated to covenant and those dedicated to the world. In this context, Koester considers the role of slavery as mentioned in Revelation 18.
Revelation 18 contains the fall of Babylon and the lament of the merchants, kings and sailors over the loss of wealth, comfort, merchandise and trade. Listed here are many different goods (which are no more absurd than are iPods, Wiis and such), over which these groups weep and mourn (vv. 11-13). Clearly implicated here is Rome AND any economic system which places commerce as the most important ideal. Though not specifically mentioned in this article, it is significant that John includes "human beings sold as slaves" at the very end of the list (v. 13). Though some may read this as climactic, it also has a natural reading that slaves are the last to come to mind; it is a sign of the dehumanizing of the practice itself. And this reads well with how Koester does build his discussion (cf. 771: "Nevertheless, [use of the term 'bodies'] does suggest the demeaning quality of the slave trade).
The perspective which is given in this article is that ". . . the slave traders not only sold the souls of human beings (Rev 18:13) but sold their own souls as well" (775). And this is a lynchpin, not only to Koester's stated thesis, but to the whole of Revelation itself. For those who are willing to participate in the ways of the world over and above the kingdom of God are forfeiting their own souls. John is bringing the force of seventeen apocalyptic chapters to a very strong statement (not that it is all for, nor does it hang upon, this one verse).
Koester spends the bulk of the second half of his article (776-785) in an historical survey of the Greco-Roman perspective and cultural milieu of slave trade and the traders. We get a glimpse here of the way in which this practice was an important part of the Empire's commerce, and how these individuals were able to achieve some status and standing. It is in this respect that these individuals further forfeit their own souls as they deal with the souls of other humans. Thus, there are two levels of implication set forth here.
In conclusion, there is much force in Revelation 18:4 ("Come out of her, my people") which is given great significance to our world today. On a pastoral level, the message of Revelation 18 is not simply and easily ignored because our culture does not endorse the slave trade, for the message is inclusive of commerce and economics in general when they violate and stand above the gospel message. There are two purchase-events in Revelation: the purchase of slaves and the purchase of redemption given through the blood of the Lamb (cf. 786). The ongoing work of the church is clear and set before his people.