Yes you can. I sold my copy of winxp which has been previously activated. Here is the copy/paste from my post:
EULA states the following:
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Transfer to Third Party. The initial user of the
Product may make a one-time transfer of the Product to
another end user. The transfer has to include all
component parts, media, printed materials, this EULA, and
if applicable, the Certificate of Authenticity.
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Here is the phone number to Microsoft Activation: 1-888-571-2048. Confirmed by calling them, that there is no problem with doing this. Feel free to call and verify. The only thing is, when the person I sell my copy to goes to activate it, they will be prompted to call the 800 number. They just need to tell the activation person that they bought it and then they will get a key to enter manually that will activate the copy.