According to David Holley a voluntary exchange occurs only if the following conditions are met (MVS p.149a):

1. Both buyer and seller understand what they are giving up and what they are receiving in return.
2. Neither buyer nor seller is compelled to enter into the exchange as a result of coercion, severely restricted alternatives, or other constraint on the ability to choose.
3. Both buyer and seller are able at the time of the exchange to make rational judgments about its costs and benefits.