@RorrKonn:  This question gets asked fairly often.  I asked it once
myself, and found the answer frustrating.  But now that I've been selling
some stuff (at 3dCommune) I understand why there's no general
answer, even within one category.  

Specific example: my 1950 Kitchen set has sold 5 times as many
copies as my 1930 Bathroom.  Both belong in the same category
(historical house interiors) and both have roughly the same number of
parts.  So why does one sell much better than the other?  Who knows?

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