For example: If the domain name DomainSherpa.com is registered with Moniker (the registrar) and the domain name reaches expired status, within a few days of expiring the domain name will be listed at SnapNames.com (auction house partner to Moniker). Domain names are exclusive to one auction service, as an auction cannot take place at two locations.
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.
A brand new domain, for instance, will have to wait for a few days before getting indexed by Google and other search engines, and then its owner will need to go through the tedious process of search engine optimization, which takes time and financial resources. High authority expired domain names on the other hand, offer a short cut and allow you to get down to business straight away.