GunBroker.com is now detecting if users are connecting to them through Virtual Private Networks (VPN), proxy servers or Tor. Users who are detected using these services are being given one warning to stop using these to access the site or have their account terminated.
These services hide the physical location of their users. If an internet user in London, UK uses a VPN or proxy server hosted in Dallas, TX, if will appear that their internet connection originated in Texas, not England. Tor, is a more sophisticated version of this, bounces connections around the world to anonymize them, making locating the user near impossible (for anyone but the NSA).
There are many legitimate reason to use these systems, especially if you browse gun related websites. If your school, college or place of work block gun websites, a VPN or Tor can get around this restriction. Many hotels and motels also block access to gun websites. I know many US troops around the world use Tor to read TFB.
On the other hand, these services can be used by fraudsters and hackers to prevent, or make it difficult and time consuming, for law enforcement to track them down.