I've said this before, but if you are into D&D for investing, you'd do a whole lot better selling your stuff and going with the stock market as long as you had a minimum of three or four years for your investment to go up and down. You avoid wear and tear, inflation and the risk of damage or loss among other things.
I'm not saing you can't make money by investing in D&D stuff, but the risks versus the gains just don't make sense for me. There are certain D&D items which have multiplied in value overnight and there are years when the stock market did poorly(two years ago), but when you look at the big picture for both, the stock market should come out way ahead over any span of five years.
The only things I collect are those things I enjoy reading and using. I collect what I like and rarely for what I can gain. I do make purchases of lots or items I know I can turn around for a quick profit, but they aren't items I'm holding onto for long.
I am in the process of reading all my comic books. I plan of selling off a bunch of the more valuable ones two or three years from now. I'd like to wait for the market to rebound at least a little, and I'm hoping that might happen. If it takes ten years, I'll wait that long. Anyway, as I finish off a set or two, off they'll go to the eBay monster. There are some sets I'll save and pass along, but mostly I'll be passing along the money.

Truth is worth finding and life is too short to work for money.