cquire A Large Diversity Of High

CAVEAT:  Some battle-tested warnings and common fraudulent tactics in the pre-existing domains market:
PR cloaking.  A lot of PR reported of existing domains is fraud from cloaking:  sending an unrelated website's high PR to another that is for sale through a temporary 301 redirect.  After the sale, the 301 redirect is removed, and the sold website has no PR.  Higher PR websites with traffic can sell in the thousands, leaving a buyer holding an empty PR bag.
PR Juicing:  the owner provides high PR backlinks to the domain for sale, that he later removes, leaving the domain without backlinking juice, even though it may show high PR until Google publishes it next quarterly PR rankings.  (If the backlinks are not currently active, no actual PR backlink juice will flow through, notwithstanding the outdated PR score.) 
PR Decrease:  The reported PR can be one quarter old, since Google updates PR ranking quarterly.  Research the current backlinks to make sure there is currently enought backlink juice to justify the reported PR. 
Protect and Build PR:  there is a natural decrease in backlinks on domains for sale due to the removal or neglect of the orginal content.  Keep as much similar content as is practical.  Try to bring back any backlinks that were reported, but are not current.  And build backlinks to maintain, or even increase the webpages's PR.
Seperate Class-C IP addresses:  host your dedicated websites on seperate Class-C IP addresses.  Google will decrease the backlink juice comming from the same IP address as it does with inter-website navigation links.  Also, have the registered owner of each high pr backlinking site be different, with a geographically diverse addresses to show diversity of backlink origination.
Acquire A Large Diversity Of High-Trust Web 2.0 Backlinks

One easy way to determine the value of a website in Google's eyes is the Page Rank Google assigned it on a scale of 10 with "n/a" assigned to webpages not yet assigned.  Getting a backlink from a website with a high PR is good. Engaging in available web 2.0 activity in high trust, high PR websites that permit the user to impute and control content on a user generated webpage hosted on their website is better.  And getting a large volume of diverse high trust, high PR, in-context relevant content with anchor-text, do-follow backlinks is golden.   

For instance, worldcat.org is a rare Page Rank 9 world-wide online library directory which encourages user interface with web 2.0 interactivity.  They let you sign up for and create a page on their website!  They allow you to create two, anchor-text, do-follow backlinks to your website.  You can enter your strategic search terms and related terms while describing yourself or your company in their biography and "interests" sections, which will show Google the backlink is in-context of your website.  When you are finished with your entries, they create a page dedicated to your site, filled with 2 anchor-text backlinks, with as much content related to your Strategic Search Term as you want to include.  

What's more, you can fill out book lists and other activities related to your Strategic Search Term and have links from other pages on their site flowing PR to your page, and on to your website through your backlinks on your page you created on their site. 

This is a highly optmized backlink because:

• from a extremely high trust, high PR site;
• with multiple anchor-text backlinks;
• with inner page PR flowing through a web 2.0 website back to your site;
• created with highly relevant, in-context content related to your Strategic Search Term;
• happy to have its other visitors to their site view and share in your interests;
• what Google loves as legitamate, pertinent, backlinks from a high authority site.