Rules are rules, fine, but when I see flipping in the classifieds I'm inclined to ignore it because I think it's actually good, not bad, for the community. Flippers are essentially providing liquidity. It's true, if you buy a pair of headphones in a sale for $100 and then sell them for $150, you might've prevented someone else from getting in at the sale price. On the other hand, the person who paid you $150 for them has demonstrated that he/she probably values them more than the person who would've paid the sale price.
There's a role in financial markets called market maker. A market maker quotes both a buy and a sell price on a financial instrument, and at any moment is buying from some people and selling to others. The sell price is slightly higher than the buy price, so the market maker makes money from the spread. Naively, a cynic might think that this person is skimming from the markets and not contributing anything. You could apply the same argument to financial instruments that Stillhart did to headphones to say that market making should be prohibited, and we should just hope that the buyer and seller eventually find each other so the buyer gets a better price.
In reality, though, the presence of market makers keeps prices uniform across the market, it reduces the cost of each transaction, and it ensures that anyone who wants to can buy or sell an instrument whenever they please. Before the presence of widespread electronic market making, transaction costs were a lot higher and markets were less efficient.
In head-fi terms, if you flip a pair of headphones, you've (1) provided someone looking to sell with a buyer at his asking price, (2) provided a buyer with a pair of headphones at a price that's agreeable to him, and (3) made sure those headphones ended up in the hands of someone who values them at what they're worth. In summation, you've provided a service to all parties involved and received a small compensation for it.
Head-fi likes to think of itself as a tight knit community, and turning a profit within it might seem unsavory. I think this is a misguided reaction, though. As a general rule of thumb, if you're buying from willing sellers and selling to willing buyers, and making money at it, you're doing something of value. Sometimes this principle is too broad, but the basic tenet of capitalism is that this holds most of the time, and in this case I think it applies.
If you are going to prohibit a type of transaction on the classifieds, it ought to be because the type of transaction is either bad for someone in the community (buyers, sellers, or maybe vendors), or belong somewhere else (for instance, vendors shouldn't be using the classifieds for free advertising). As I said above, I don't think the former is the case. As for the latter, flipping is not a viable business model, so there's no chance of the classifieds section becoming overly cluttered by people trying to make money. And so what if it did? Even if flipping became very prevalent at head-fi, the classifieds would remain a place where you go to buy or sell used headphones. It would just be easier and quicker to find a buyer or a seller.
So, yes, it's against the rules. But I wish it weren't.