Buying a domain name can be a tricky process – especially when so many names are mostly already taken. Thinking of a truly unique name (Zoopla and Flickr being obvious examples) can help you build a strong online brand which is also helpful for SEO purposes, but if you want something more specific then you could be tempted to buy an expired name.
However, buyer beware! Expired names can have a history, and be banned from Google for being implicated in Black Hat SEO techniques. So Matt Cutts has recently issued some tips to help the unwary avoid buying a dodgy domain.

Before purchasing any domain name, Cutts advises buyers to run a search on the name first. If no results appear and the domain has been previously used, this should be a red flag. Secondly, you should do a search without the .com, or, which can show you any potential problems with the reputation of the domain. If there are comments about spamming then leave well alone.
Finally, you could check for previous versions of a site, and again check for spamming. If you do happen to buy a dodgy domain, then you can ask Google for a reconsideration request. If the links are not useful to you, you could also try disavow to wipe the slate clean.