Surely if you purchase a domain ( and redirect it to your existing domain ( you expect to loose all ranking for If 301's are used then the objective is simply to pass on authority, not dominate the search results with 2 domains. If you want to keep in the index then you would take Rebecca's option 2 or 3.
2) prerelease auctions.  These are domains where the auction venue has contracted with the registrar, like Fabulous or Moniker, to auction off non-renewed domains.  Just before a domain enters the pending delete process, it goes to a private auction to any person who has backordered the name.  Some people feel that back links on these type of auctions still provide a SEO benifit. My testing indicates that there is zero SEO benifit when these domains are redirected into another site.  So if you get a domain with "pink squirels" in a lot of achor text to a domain and then point it at your domain, you don't rank for "pink squirels"
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