I don't think the long-term aquarist is greedy at all. The fly-by-nights, sure, but I don't even know if I'd go so far as to call them "greedy", but instead say that they are just as clueless about resale value as they were about the hobby they decided to jump into and quickly leave when their tank crashed.
On CL, in my experience, you're almost exclusively going to find the short-term aquarist. They didn't get into the hobby enough to find forums and local clubs to sell their stuff at so they take to the only resale outlet they know - Craigslist. They're the ones trying to sell their 20 gallon long with plastic plants for $300 because that's what they paid at Petco three months earlier.
The long-term aquarist, first and foremost, knows that a used skimmer, regardless of how much it was new, is only going to be worth about half of what they paid, if that, when reselling. More than that, the long-term aquarist is likely only upgrading or downgrading instead of chucking the whole hobby and so they tend to hook up other aquarists when we do buy from each other. For example, I bought some frags off a local reefers, got good prices AND a free frag - that's how we roll; helping others who are either new to the hobby, or who are essentially 'trading' equipment and livestock (I say 'trading' because odds are we've all bought AND sold any number of things so we are essentially just swapping the same money for goods and vice versa back and forth all the time).
I think it is extremely rare to find true "greed" from a long-term hobbyist.