1. The standard expression is commodities market: convention of usage determines in this case that the the plural noun serves as attributive modifier.
2. I would say that none of these expressions has any kind of standard currency, but if you required a phrase to denote a document that lists certain requirements, then it would naturally be termed a requirements document (structure as above).
3. Farmers' market: the plural possessive-case noun (on account of the human referent in this case) would most naturally serve in this case.