大部分情况下应该归因于商品的价值（抱歉我不得不用这个容易引起争论的词）随着时间的流逝而下降吧。比如易腐食品，报刊杂志等。

当然火车票（包括各种类似票证，比如演唱会门票等）的情况特殊，这种商品有一种特征，就是它们有一个有效期，有效期内价值不变（提前一小时上车和提前一分钟上车是一样的），而一旦超过这一期限（比如停止检票后），这一商品就一钱不值。

总结以上论述，想象一个时间为横轴，价值为纵轴的坐标图，可以说如果报刊杂志等商品等的价值随着时间增加而呈线性下降的话，火车票等商品则有一个“转折点”，在转折点前呈一水平线，过了转折点则变为零。

正是由于火车票过了“转折点”就一钱不值，而开车时间越近，票越可能卖不出去，所以票贩子愿意用票价的损失去抵消卖不出票的风险吧，呵呵。

[此贴子已经被作者于2007-12-26 21:15:04编辑过]

我可以传文献给你看看。

    Many new products, like the successive generations of pocket
calculators, are very expensive when they first appear on the market;
then, over a period of time the price declines. Similarly, new books
often appear first in hardcover and later as less expensive paperbacks.
Although-in some cases the falling price can be attributed to such
factors as decreasing production costs or the introduction of tom-
peting products, in other cases it appears to be, at least in part, for the purpose of exploiting differences in consumers' reservation prices.
That is, some of the price cuts appear to be for the purpose of price
discriminating.