Some airlines, like Southwest, don't sell on the consolidated sites so you need to go to their website. It also depends on where you are flying. There is a flights forum on cheap european travel website that posts lots of good deals. For instance, someone posted Aer Lingus flights from Chicago to Dublin round trip for $234 a couple of months ago. Couldn't find those prices on the consolidated sites. A couple of times I've found excellent prices on CheapTickets.com but you have to deal with a small goofy travel agent in NJ.
I've also found that buying too early can result in high priced tickets. As I recall, flights to Europe for the summer should not be booked before the end of January.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13