The guy who fixed my roof told me to purchase a minimum of 10 keyword-rich domains for various aspects of my business. What's the best use of these for search engine optimization purposes?
Unfortunately, there's no good use for those when it comes to the search engines. If you try to gain an unfair advantage in the engines by blanketing them with lots of different sites, you will then be into that "deceptive" territory I'm always talking about.
In my opinion, your best bet is to simply park them all at one domain, and then forward the whole lot via a permanent 301-redirect to your main site, while telling your roofer to stick to roofing. ;-) (See "How To Redirect Multiple Parked Domains.")
The only other viable option for the domains would be to truly build completely different sites at some of the domains. There's nothing wrong with having a few sites, but they really should each provide different info in order for them to be effective. Just remember that keeping and maintaining many sites is a lot more work than just having one site that is the best it can be!
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