The chief costs of spam are the time and attention of people who don't want it but get it anyway. Those costs are borne neither by the buyers nor by the sellers but by third parties.
Labelling externalities costs in the traditional sense is misleading. By the same logic one could argue that the "real cost" of meat-production isn't borne by the buyers or sellers of meat, but by the moral-vegetarians whose conscience the process bothers. Similarly, just because Pepsi has billboards and I prefer Coke, doesn't mean I would say Pepsi existing delivers a cost to my time.
Misleading products on the other hand are different, (when the offer isn't genuine), but spam does not necessarily mean fraud.
http://en.wikipedia.org/wiki/Externality