Tamriel Trade Center and all other addons that contain exe files are in no way different and pose the same risk as Nirn Auction House.
To be honest, I think it was a bad idea to allow addons with executables at all. Instead they should have been forced to use scripts (lua, js, php, python, whatever) to handle what they try to accomplish. The end users would have to install an interpreter on their machine, but at least that way it would be very hard to hide malicious code compared to when a precompiled binary is used.
Maybe for the future, you could add some sort of reputation system? New addon authors without a reputation are not allowed to upload binaries. And when someone tries to download an addon with a binary from someone with low reputation they get a big warning telling them that it might be very dangerous?