I am a bit concerned that many out there are not familiar with basic economic principles. I've taken several university classes in economics, and I have learned that second-hand markets nearly always balance themselves. Arguably, the most important lesson in economics is that supply is inversely related to demand. As it relates to this conversation, we are dealing with a fixed supply (clearanced or out-of-production items). As demand increases (more people wanting it) beyond the existing supply, there are not enough items for everyone to have what they want. Naturally, price increases and becomes the determining factor between who gets one and who doesn't. Tell me, if price doesn't differentiate, then how should the second-hand market decide who gets one? I know no other fair way. The person willing to out-pay the others takes it home.
I also need to mention that sellers never rip people off with high prices. If someone is willing to pay, then it was priced at the level of demand. If no one is interested, then the seller was overcharging and gets no sale. Either way, no one was taken advantage of. This is not a case of monopoly (like a power company) or a necessity (like food or water). There are always other buying options, even if some are currently not on the market. If the value becomes too inflated, more sellers will join in, lowering price back to a reasonable level. Also, competition allows sellers to undercut each other if they are all overpricing until price reaches demand again. As I said, the markets balance themselves.
Having said this, I will agree that "hoarding" is inconsiderate. This is why I buy the majority of my stock online, so that at least no other local customers or kids get disappointed by an empty shelf. Remember, though, that even those who buy up the stock of current products are only helping TLG, as sets that are still in production then get extra orders processed, and more sets get made to keep up with the demand. Who could have a problem with extra $$$ and success for The Lego Group?