Some collectibles plummet, others remain about the same value, and some just continue to go up with age. If I am considering investing in collectibles, what are some factors I should be aware of that influence how well different kinds of collectibles hold value?
Factors that affect value of a collectable are how many copies remain in circulation, what condition it is in, whether there is any (provable) special history associated with it, whether it is actually useful or interesting for some reason, and -- most importantly -- whether anyone actually wants it.
If you're interested in collecting for your own entertainment, go for it. (I collect first-edition paperbacks from one of my favorite authors; I'm still trying to complete the set.) If you're thinking you'll make money at it -- well, AT BEST that's a game for experts, and you should remember that if you look around a poker table and don't see the sucker he's sitting in your chair.
When a collectible is placed on the market but does not attract a sufficiently high price, i.e. the secret reserve price, it may be pulled off w/o any sale. Therefore, stories about high priced sales are not representative of the whole class of items.
IOW, it's impossible to tell how the collectibles market is doing because only the items that are going up are mentioned.