According to my very new experience see here, the kind of market place I dream should not be only for second and phones but also for spare parts. And also for special needs.
The success of the fairphone is not first from technique, but from the concept of fairness where a company and a community are linked.
The success of a fair second hand market place would be not on having the best price, the best material, but the fairest one according on ideas of solidarity, sustainability, no-obsolescence.
People looking for the best price know the best places for that (ebay…). People looking for the best technique know the places for that.
The best opportunity a market place on fairphone.com could offer is about fairness assumed as long as possible by the community.
Yes rules must be kept to preserve quality, credibility, but the aim is elsewhere, further, deeper.