Since the quantity and quality of links pointing to your domain has a large impact on your organic search engine rankings, many enterprising website owners have thought about buying expired domains so as to capture existing links pointing at that domain. They may set up a site there, or redirect it to their original site. Does this work?
The answer is “sometimes.” If you buy a site that’s up and running, and keep it as is, absolutely. If your company merges with another company and the domain is transfered within a corporate entity, probably. If you buy a domain from someone before it goes back into the “common pool,” possibly. If you buy a domain after it goes back into that pool, not likely. If you buy a domain and redirect it, not likely.
That’s it in a nutshell — for more details, read “Do Links From Expired Domains Count With Google?“