they have always the chance to sell it, even for a low price
That's true the_poet, although many people aren't able sell their expiring domains, even for cheap.
For example, I vaguely recall Jeff writing about how he tried selling some soon-to-expire ccTLDs very cheaply, but there were no takers. The ironic thing is that after the domains dropped, a popular drop-catching company picked up the domains and only then did people start bidding on the domains! Now that sucks.
I've always thought auction system has some psychological factors in it that influence people and let them believe they could have the domain for just "1 more dollar" over the current highest bid. Because nearly every bidder makes this kind of reasoning, the price usually goes up fast in auctions.This is a strange occurrence in our business, names that could not be moved for reg fee on forums appear on places like snapnames and suddenly the feeding frenzy starts and they are at $xxx even $xxxx.
The auction scenario seems to have a lot to do with it and probably considerably more potential buyers with $'s than on the forums frequent the auction houses.