Personally, I would never sell a for more than I paid for it. Like n.maher, if I really need cash and had to sell something quick, I would offer the item back to the original seller. If they were not interested, I would pass along the savings to another Head-fier. If I was going for all out profit, I wouldn't post the item here, I would post to Ebay.
I have no problem buying and selling over time (no flipping though!) and have done so myself (having owned a bunch of hp-1000s trying to find the best one I could and then eventually selling the one I had settled on years later for a phono stage) but I HATED those that bought the HF-1's and then flipped them as soon as they were no longer offered. Some folks were putting insane price tags on them immediately and it just ruined the entire gesture for the community.
As was mentioned before, this site is not a market place, it is a community first and the FS forum is a privilege for members in good standing who have demonstrated some form of commitment to the site. This site is very unique among online forums and even more special among audiophile sites. Stereophile along with TAS and others are awestruck with the membership here, the commitment to the hobby and most of all growing our meets to something annual and international.
We have manufacturers who make exclusive products for our site and then donate the profits to charity, thereby covering the parts cost and labour costs and awarding the members appreciation of their products over the years with something unique.
Although not illegal, there is always something beyond rules and regulations, norms and laws...these should be founded on values and values stem from acknowledging the dignity of something or someone. In the case of getting a sweet deal and then flipping, I would say it should never be done. If someone received a sweet deal and simply wanted a change after a time, and contacted the original seller regarding this, if they sold at market value, then I wouldn't be disgusted with the person, I just wouldn't do it.
The idea that one must sell at market value as a means of benefitting the community is hogwash at its finest. A perfect example are the HP-1000's, clearly an over-priced headphone yet people keep bumping up the price to insane levels. I'm waiting for the bubble to burst, and it will and all those folks who paid near 2k for a once 595 phone will feel pretty silly. I'm not against capitalism, but I am against insanity and promoting it as a benefit.