This is where it all went wrong. Because, very frankly, it's a commodity much like stocks. The market goes up and down, and *you have to know what you're doing* to a point where you could work for a museum or as an appraiser. You have to read *all* the catalogs you can, go to the trade shows, talk to dealers all over the world, and network, network, network. Or you'll lose your shirt.
I'm at a point where I can write books on my fields. (I'm working on a couple presently, if I ever have a life again.) I've been writing (and selling) articles on book collecting for almost two decades. When I talk about cookbooks as an investment, I'm not talking anything published in the past 20 years. I'm talking pre-1930, and you have to know the stuff inside and out, in terms of both value and condition. Never buy something for list value (unless you really want it for other reasons, like you need the information--and then don't consider it an invewstment). And then there's condition, condition, condition. Most books folks think are worth "a lot of money" are almost valueless, and it's all because of condition. You can sell a 1995 book club selection for $12, while an 1870 book with water stains and missing pages will go for $5.
Then there's vintage and antique clothing, where it's even more sensitive, and you have to know the market within 6 months to a year, and also know about regional markets. (I'd love to buy stuff in Oregon and sell it in New York--instant 1000% profit.) And I can I.D. a 19th century dress within 3 years---something most dealers can't even do. Kimono---I can generally place them to era (Meiji, Taisho, Showa (early, mid, late), "modern" etc. Which is *really* hard for non-experts. (I've caught misidentifications in the Cristie's London auction catalogs!) But it's a complex field, and you've got to live and breathe it. And the mistake a lot of folks make is *wearing* antique clothing, which is as good as trashing its investment value, right there. I'll also admit to selling things because my spidey-sense said "this is going to get silk rot in 10 years" and wanted to eliminate it before it lost value. And that's more of an "educated fingers" intuition that you have or don't. (Yes, I've written articles on antique clothing, too, back when I was writing for the "antique lifestyle mags" like Americana and Early American Life.)
But the bottom line with average folks: only buy what you truly love, and don't worry if it goes down to nil in value. Which is how I ended up where I am now---30 years later.