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If you want an accurate idea of a machines popularity (and subsequent demand) have a look at the top 100/300 lists produced by ipdb and pinside. They provide an accurate gauge of a machine's ranking along with feedback from a broad cross section of the pinball playing community.
http://pinside.com/pinball/top-100http://ipdb.org/search.plAs pinnies4me/retropin said, not a lot of demand for Sega and Data Easts compared to Bally/Williams.
