Resale of items occurs quite frequently in the world. Look up “garage sales”, for example.
The line becomes blurred, however, when you’re trading in digital assets that are not physically tangible in “ownership”. That is, when you’re “licensed” by terms that you agree to when you purchase a product and use it.
The “legality” will be determined by courts (internationally or locally) and will likely also be referencing these terms. Blizzard could seek damages based on the trade of property which was not “owned” by players, and/or sites which facilitated those transactions if they so wish.
In summary, trading in physical property RMT is quite different from trading of digital IP, especially with the precedent of laws (DMCA, etc.) which have been enacted in the US.
Anyone remember Napster?