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Marc S.on this board who collect cards of every player, so have developed pretty extensive lists of those players who may have only appeared on a single issue. However, those lists are not widely available or distributed...as many, many hours of hard work has gone into creating them, and the tendency to distribute them only adds to the competition of adding those players to others' collections. It may not be surprising to you that those players with only a single card appearance (especially those with MLB experience) tend to command a premium valuation, all else equal.