E.g. toys - there's a thriving market in Lego minifigures. In fact, sometimes the minifigures can individually fetch more than the set, because for many of the ranges, all the sets will contain different subsets of recognizable characters, so there's often a market consisting of people that have e.g. 3 of the 4 ninja turtles and want the last one without paying for a full set. So the prices are already ridiculously high for some items.

But many of the people in that market are totally unsophisticated. For example parents selling of their kids collections once they've "grown out of it", and when they misspell something, items can go for 1/4 of their market value or even less...

If you're willing to hold on to them and monitor prices for a while, you can do even better, as many of the ranges appreciate substantially in value (big collectors market...).