Every time a new buyer comes along and buys an expired domain with traffic the original search engine traffic vanishes and it is because you don't have the same content as the original site. Generic words will always be able to rank again but brandables require all the effort to have them ranked. Buying domains for their past ranking can be a waste of time as can be difficult to simulate to obtain the pr ranking. If buying used sites that rank try obtain the content if wanting to ensure any rankings.
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