I was a user of Photoshop CS6 but decided to subscribe to the CC for Photographers offer last year. The offer required ownership of a previous CS package so I was fine. However, now I'm using Photoshop CC and Lightroom I have no use for CS6.
I could sell on CS6 and transfer my ownership of it but would this prevent me from continuing my current subscription into a second year? It seems a waste to have it hanging around if I'm not using it and Adobe no longer require to see it for the subscription.
Welcome to the club. We all have these old versions lying around.
You can't legally sell it if your current license is in any way tied to it.