My dad was heavily into the whole collectible/antique thing, and I picked up a fair amount of the basics from reading the books he had and helping him sell on eBay.
You have to be careful, because we’re dealing with three entirely different categories here:
Antiques are furniture, toys, household items, advertising…pretty much anything that was made prior to 1900 and is still in decent shape. Not everything that makes it this far in time is worth more than it used to be, but if it’s managed to endure well enough to still be usable/presentable, it usually has appreciated.
Genuine collectibles (Collectibles 1) are items that weren’t originally manufactured for the purpose of being saved or collected. Vintage toys (less than 100 years old), kewpie dolls, utilitarian tableware (like Depression glass), and objects like that fall into this category.
What I call devised collectibles (Collectibles 2) are the last category. These are items that were specifically manufactured as collectibles. Franklin Mint figurines, those painted Liberty half-dollars, Gone With The Wind plates, and all of that type of thing belong here.
Items in the Antiques category are, within reason, always appreciating as long as they retain a majority of their original makeup. The Keno brothers on Antiques Roadshow are always bemoaning a refinishing of a fine piece of furniture because it really does lower the value. As long as the item isn’t seriously damaged and hasn’t been radically restored, it’s going to keep going up as long as there are serious antiques collectors with money.
Items in the Collectibles 1 category go through cycles, but the dips and peaks aren’t too far away from each other. They have value because of rarity, nostalgia and a sense of camp, not for any inherent superiority of workmanship or material, but the arbitrary value is fairly stable because the rarity of these pieces is NOT manufactured, as they were never intended to be an investment in the first place.
Items in the Collectibles 2 category, on the other hand, are as volatile as gasoline. An awful lot of people are holding an awful lot of monetarily worthless pieces of glass/metal/plastic/paper because they were led to believe that this item or that item was the next big thing by the manufacturers who make them. I know I’ve seen Franklin Mint figurines on flea market tables selling for $10 or less. Not a good investment.