Not just the problem of fakes being abundant...
Hidden mickeys are lower dollar pins. You buy a booster pack and go wander the park trading for hidden mickeys.
There are folks who collect the sets. Mostly though, they collect the current year, complete the set, get the sense of accomplishment then let them go for the next round of releases.
I have noticed people like the completers, the ones you purchase via mystery pouch for $2.95 when you buy more than $30.00 in pin product at a store. Those tend to be more sought after and valued a bit higher.