[edit: apparently as I was writing this others made a lot of the same points, so sorry for the repeat!]
The economics major in me agrees with Sumi. It's just two parties doing a transaction. What price the seller is willing to sell for is his choice, and it's up to you whether you want to accept it. The past is the past; doesn't matter if he acquired it for free or paid $1 million for it.
Compare swapping gear to the stock market, where buy-low sell-high transactions take place all the time. One person will buy a stock at a certain price, say $10 per share, and then sell it for a higher price later on, say $20 per share. No one I'm aware of questions whether this is ethical.
HOWEVER I think Justin brings up a good point about this being a community. It's not the stock market or any other typical market. And personally, buying low from one BPLer and selling high to another BPLer strikes me as pretty sketchy. It doesn't sit right in my stomach.
But circumstances can vary, and I think intent matters.
If, for example, someone came in here and bought a bunch of 3-season tarps and clothing in the dead of winter (when demand for 3-season stuff is lower) without the intention of ever using them, and then sold them at a profit in the summer (when demand for 3-season stuff is higher), that might strike a lot of us as unethical.
In comparison, if someone buys, say, a stove 10 years ago for $25 and now it's a rare and coveted stove, would it be ethical for him to sell it for a profit? Personally I would have no qualms whatsoever with that.
Even in the first example, where someone is buying stuff with the sole intent of profiting off it, I think there's an argument for it being ethical. The guy is providing a service to the seller by taking it off their hands, and providing a service to the buyer by selling them something that's worth it to them at that price.
For example, say I posted a tent on gear swap this week and really need the cash. I wait a week and no one is biting. Then some guy comes in out of the blue and buys it. I'm relieved. I got the cash I needed to buy those new winter boots I wanted. Fast forward six months, summer is here. Guy who bought my tent hasn't used it once and posts it on gear swap again. He ends up selling it for $100 more than what he paid for it.
Is there anything wrong with that? I'm happy because I got the money when I needed it and no one else was biting; he's happy because he made $100; and the buyer in the summer is happy because they got a tent they wanted when they wanted it at a price that was agreeable to them. And put yourself in the poor gear swap profiteer's shoes. He lost the opportunity cost of the money he paid me for the tent and took a risk. He had no guarantee that he'd be able to sell the tent for a profit.
That being said, it still strikes me as kinda skeezy to take advantage of the goodwill of community members who are willing to sell their stuff at a good price for the sake of giving other members a good deal. So think what you will...