There are, surely, other possibilities.
The first different possibility is that shadow brokers really hacked the server and obtained the NSA hacking tools. They are definitely Japanese data brokers, but they might have more hacking capability than originally assumed. Although it is quite doubtful they were able to hack the NSA, but they might obtain the data from the server of the equation group.
As a matter of fact, they mentioned they hacked the equation group, which is possible. If the equation group has halted their activity since 2013, it is likely that they should do so with some reasons. If that is the case, there is a probability that their stolen hacking tools have been hidden in the certain server without enough maintenance.
Under this condition, the shadow brokers might hack the equation group to obtain the tools. This is just a slight possibility but they might obtain them by their own effort, not from the global crime organization.
In this case, their purpose was to show off their capability to the US government, and simultaneously to the equation group. There was no other practical reason why they conducted the crime in this case, which implied the auction was too much, therefore this possibility is unlikely.