This serial number is already in use.

[reliable_Sunflower16B8]

I recently purchase a second hand licence for an Adobe Master Collection CS6 upgrade for my existing Adobe Master Collection CS5 licence.

This seems to have worked well.

However, when I go to my Adobe account and try to register the new licence I get a message:

"This serial number is already in use."

Is there any way I can resolve this?

[LinSims]

The person who owned the license has to transfer it to you. 

https://helpx.adobe.com/x-productkb/policy-pricing/transfer-product-license.html

[reliable_Sunflower16B8]

Many thanks for your posting - that was very helpful.

The instructions you provided specify that a Case Number must be provided for the transfer to take place.

How do we obtain a Case Number?

[LinSims]

Looks like filling in and returning the License Transfer form gets you a case number, but I don't know. Like most of us around here, I am not an  Adobe employee.