Assessing the sustainability of the harvest of animals can be done by obtaining data from processing facilities and establishing that vital attributes of the harvested animals (e.g., size, age structure, sex ratio) do not change over time. This model works if the traders operate in a free market without any regulations on what can be harvested, processed or exported, and when harvest methods and harvest areas do not change between assessment periods. Several studies assessed the harvest effects on blood pythons (Python brongersmai) in North Sumatra, Indonesia seemingly under a free market scenario, with some concluding that trade was sustainable and the others hinting at an overharvest. Indonesia has established harvest and export quotas and, internationally, trade in blood pythons is regulated through CITES, and the blood python trade clearly does not operate in a free market. Data suggest that the three (or four) slaughterhouses included in these studies processed ~27,000 blood pythons a year against a quota of 18,000. There is a risk that data from traders alone purporting to show that harvest is sustainable will lead to an increase of quotas or an abandonment of quotas altogether. There is no conclusive data to support that the harvest of blood pythons in North Sumatra is sustainable but there is sufficient evidence to suggest that a substantial part of this trade is illegal. Likewise, at a global level there are clear indications of misdeclared, underreported and illegal trade involving 10,000 s of blood pythons. While important biological information can be obtained from harvested animals, to assess whether harvest is sustainable there is no substitute for monitoring wild populations. After decades of international trade in blood pythons from Indonesia, during which at least half a million blood pythons were exported, it is all the more urgent that systematic monitoring of wild populations commences.