I had an HTC Wildfire phone which I stood on and broke the screen. I then sold it as spares because it wouldn't turn on and bought another one. When I went onto my Market account I noticed my new one is there and so is the one I sold, both registered to my Google mail address, and there is recent activity on both of them. I could not reset the old one to factory settings before I sold it because it wouldn't turn on. Is there a way I can unlink the old one from my account? Its beginning to worry me that someone else is using it through my Google mail account .
If you go to the Market Account settings part of the Android Market webpage at https://market.android.com/account and click on the Settings tab, you should see a list of the devices registered on that Market account, clicking the Edit button lets you hide that old phone from the web interface. That's what you'd normally use to remove an old device from showing.
If you're worried about whoever renovated your phone using your account, then you need to go a step further than that. The easiest way to block the other phone out will be to change your Google Account password. Then your phone will prompt for the new password next time it connects to a Google service, as will your old one, but as only you know the new password it should stop them from using your account on the old phone.
To stop this sort of thing happening in the future you ought to think about setting up two-step authentication on your Google account, once you've got that set up you can log into your Google account's settings and revoke the access password for any phone or device one at a time once you don't want them accessing your details any more.
Change your password then revoke it's access.
Also, start using application specific passwords and 2-step verification if you aren't already.
This makes it easier to control what gets access where.
This should prompt who ever is using the phone to reauthorize your account (via password or 2-step), which they won't be able to.