comprasen
Imperfecto de subjuntivo para el sujeto ellos/ellas/ustedes del verbo comprar.

comprar

Esos inspectores velaban también por que únicamente se comprasen diamantes a mineros o corredores legítimos de diamantes y además debían certificar que diariamente se rellenasen correctamente los formularios proporcionados a los comerciantes por el Ministerio.
These inspectors also ensured that the diamonds were only bought from legitimate diamond miners or brokers and were also tasked to certify that the Reporting Forms provided to the Dealers by the Ministry were filled in daily and correctly.
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el espumillón