Buying used windows license. How can I tell if they are still active?

I want to buy used copies of Windows Full Retail Version, (XP, Vista, 7) so we can install our PC application on customers Macs using something like Vmware fusion.

If we do buy these licenses, how can we tell they are legit and not being used anymore? Will it tell us when we try to activate them? Are we liable if they are being used at the same time as the original owner?

Any other pitfalls to this strategy? Thanks in advance.

From serverfault muhan

• Windows 7, for example, has the following provision. This means that the seller must be the first user, and provide you with any media/collateral, and a proof that they purchased the software.

  You don't have any way of verifying that they are no longer using the software, there is no mechanism to do this.

  From the Windows 7 EULA at

  http://www.microsoft.com/About/Legal/EN/US/IntellectualProperty/UseTerms/Default.aspx

  TRANSFER TO A THIRD PARTY.
  a. Software Other Than Windows Anytime Upgrade. The first user of the software may make a one time transfer of the software and this agreement, by transferring the original media, the certificate of authenticity, the product key and the proof of purchase directly to a third party. The first user must remove the software before transferring it separately from the computer. The first user may not retain any copies of the software.

  From Greg Askew

• I believe the EULA for Windows XP Home and Professional Full Retail versions also allows for a one time transfer to a third party.

  http://www.microsoft.com/windowsxp/eula/home.mspx

  http://www.microsoft.com/windowsxp/eula/pro.mspx

  From joeqwerty

• In many countries the no transfer clause of the Microsoft EULA is not legal. In those countries you can resell or buy used Windows licenses no differently than you were buying/selling a used car. Check with your legal dept.

  Tachyon

  From Tachyon