How to deactivate sold device?

No way to manage single devices.  Unfortunately Adobe doesn't have any appropriate web app.

The only thing you can try is

Adobe Live Chat: http://www.adobe.com/support/chat/ivrchat.html,

or as a slight short cut try http://helpx.adobe.com/contact.html?product=digital-editions&topic=using-my-product-or-service

Choose topic ‘'Signing into my Account' , and then click on 'I still need help';

then you should see 'Chat with an Agent' at the bottom of the page.

Depending on screen resolution, you may need to scroll down a bit to see the Chat with an Agent' bit, just under 'Ask our Experts'.

'Ask our experts' will indeed just lead you back to this forum.

Sometimes you will get ‘Sorry! All agents are busy— please check back soon.’

Don’t refresh the page, just hang on and it should eventually go to ‘Chat Now, and agent is available’.

They can reset your authorizations, and then you must reauthorize any devices you still need.

(Unfortunately, Adobe haven’t got round to an admin website for viewing and editing authorizations.)

Some of the representatives haven't been properly trained and don't know what to do (and claim there is nothing they can do);

in that case the only way seems to be to give up that chat and try another session hoping for a properly trained representative.

If your problem is with another device using Overdrive, Bluefire, Aldiko or similar third party app, it is recommended not to mention that app when on the chat, just mention that you have run out of authorizations  (E_ACT_TOO_MANY_ACTIVATIONS) .  Thanks to AJP_Bear for that tip.