I'm looking at cars right now. Is it a wise idea to run a CarFax on cars I'm seriously interested in? Or is it just a waste of $40? Are there other sites comparable to CarFax that I should be using?

Thanks,
Ryan

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD