This method does not require you to connect to XBL with the KV to check if its banned or not. Other methods that require you to connect to XBL can get the KV flagged.

What you need:
o Xbox.com account
o The Keyvault's Serial Number



All you have to do is these few steps.

1. Sign into Xbox.com
2. Go to support.
3. Register new console. (If it is already registerd. Then scroll down and read the
4. Console already Registered?" part.)
5. Fill out the information to register the console.
6. Attempt to make a repair request for the console.


Unbanned or Banned?
o If the repair request goes through and is accepted. Then the KV is unbanned.
o If you get a error over and over again when trying to make the repair request then the KV is banned and seller is trying to scam you.


Console already registered?
o Call MS and try to get the console registered under your name.



How accurate is this?

o This is 100% accurate. Reason being is that when a Xbox 360 is banned. The console is blacklisted from being repaired. So if the system allows you to make the repair request that means that the Console the KV came from is not banned and the KV is usable online.
creds to s7