The association, FACUA, told Spanish authorities it has detected "31 announcements of organ sales" on 13 Internet sites by Spaniards and Latin American immigrants.
"These are offers of kidneys, lungs and bone marrow, by persons who say they are going through economic difficulties and asking for sums ranging from 15,000 to one million euros (19,000 to 1.3 million dollars)," FACUA said.
It described the offers as "a growing phenomenon in times of economic crisis, the result of a stressful situation for people who have lost their jobs or their businesses, who are reeling under debts or are at risk of losing their homes."
The association also expressed concern the would-be donors could become victims of international organ trafficking networks.
The Spanish economy entered into its first recession in 15 years at the end of 2008 as the global financial crisis accelerated a downturn that was already underway in its once-buoyant property sector.
The unemployment rate shot up to 15.5 percent in February, the highest level in the 27-nation European Union.