The easiest and least time consuming option is to 301 redirect the old domain to your existing site. This tactic obviously works best if both sites are in the same sector and are targeting the same keywords; otherwise, if you have a pet supply site and you buy an old Texas Hold 'Em poker site, a redirect probably might raise some eyebrows among the search engines. If, however, your site is brandnamepets.com and you buy onlinepetsupply.com and 301 redirect the domain over, you're inheriting a lot of topical and appropriate links.
Yes, I know, it seems backwards that you’d have to compete in an auction for a domain name that may never be caught. Other people feel the same way and many don’t compete as a result of that. But that’s the way the drop catching service was set up. Who knows, it may mean less competition for you since others do not like that model, which benefits the company rather than the user.
If you buy a domain from a 3rd party, the backlinks are valuable. I always wait a few months before redirecting, so just set up a quick minisite until two page rank updates have passed and then go ahead and 301 redirect. However, it seems that you'll get the link juice from the back links, the anchor text is ignored by Google. Buying a domain at Godaddy TDName expired auctions, seem to not count as a dropped domain if you immediately set up a mini-site.
1) pending delete auctions. These are domains that were not renewed and are going through a pretty complicated drop process. You can backorder these names at the auction venues and then each attempts to "catch" the dropping domain. If one is successful you win the domain at $59 if you're the only one to backorder the name. If more than one person backordered the domain a 3 day private auction is held. Once the domain goes through the drop process, the back links have pretty much zero SEO benifit to the new domain owner.
For many reasons, these domain names expire and become available to the market. Perhaps the domain renewal fees are unpaid. The owner no longer needs the domain or business came to a halt. With the large pool of domain names that become invalid daily, investors who are looking to cash in on the traffic that they generate have to do extra work to find the most profitable every day. The trick is to find expired domains.
The way this list is implemented means I can't update prices, nor removed sold domains automatically and sedo doesn't provide me with the tools I need to implement it properly. Unfortunately some users keep contacting me to update their domain prices or remove their domains and I can't keep doing that manually. So I've decided to disable the list for now.