If you're wondering why people do it on a photography forum, the answer is simple. This is not just a virtual gathering place for photolovers to share their images and lust for gear. This forum is also the gathering place for a target market.
Some enterprising individuals spend time not only buying up high demand Big Stoppers, but also purchase good deals on used gear to flip to other members. If you look at the For Sale forum, there are POTN senior members that are only active on this forum because they sell new and used gear. They made it profitable for them to be members here. For at least some of those people, it's a significant supplement to their usual income and helps them pay the bills. There's no right or wrong when there's a market for it. After all, no one has a right to purchase luxury goods at retail value.
I just purchased a 5D3 today from a guy off of Craigslist. I paid $2850 for it and the Adorama receipt shows that he must have gotten it during the eBay $2750 deal. He made a profit and got to use it at his buddy's wedding. Arguably, he made $200 ($100 profit from me plus $100 cost to rent the camera for a couple of days). Am I down on myself because I didn't get the screaming deal? Perhaps a little. Do I blame him for walking away with a smile on his face? Not at all. He was awake while I was asleep. So be it.
Yes, but that's a 4% markup... not really taking advantage so I wouldn't lose any sleep over it if I were you... the Lee markups however are about ~35% so would you still be only a "little" down on yourself if you had paid $3712 which is +35% and found out he paid $2750... I'm thinking not...