Aruna, I used to work for some antiques dealers, years ago, bidding at auctions on thier behalf. They had a reputation for spotting the rare and unusual and snapping up a bargain and so when ever they bid, other dealers would join in, driving the price up. So I'd go in, bid for items, pay with cash, and take the stuff away without anyone knowing who had bought it. I once bought a diamond brooch in the shape of a lizard for £7,200 which a week later sold at Sotheby's for £38,000. Lovely thing!
Now. There's a huge black market in antiques. There are some very wealthy collectors out there who will happily pay over the odds for something rare, or famous. They then have the pleasure of keeping these things at home and being able to look at them when ever they want. And that's what they do it for. I can quite imagine someone buying a rare stamp and keeping quiet about it; enjoying the knowledge that only a couple of people in the world know about it's existance. I've heard that paintings especially are stolen to order--it's a bit more difficult to steal a good settle, after all--and I can see how a stamp could also find itself stolen, bought by a rich collector, and then hidden away for years.
They'd have to care an awful lot about stamps, though. Hmm.
As for keeping them in good condition in a tropical climate: the problems would be moisture, mould and insect damage, I suspect. Could it be stored in an album, but with some sort of dessicant alongside it? Would grains of rice work (I remember salt-cellars in Greece always having a few grains of rice in them, to keep the salt dry), or would that not be sufficient in the climate? You'd probably need several pounds of rice for it to last over decades!