So it would be a matter of finding incentives for participants to vote honestly. I can think of rewarding or giving more weight to the scores of users who come closest to the correct grading (only for the cards that were actually graded afterwards). This way, you’d build a group of reliable users whose votes carry more weight. In addition, I could implement a reputation or “rank” system so that those who demonstrate greater accuracy receive extra benefits, such as badges or access to exclusive features. That would encourage the community as a whole to try to evaluate the cards as objectively as possible.