In theory it is a totally stupid practice. I don't get pissed off and shout at my broker when I find out that 6 months ago my stocks were worth 25% of what I paid.
It also prevents people trying to find a 'market value' price, like I did when I wanted to sell my HD497s. Eventually I just gave up and lopped $10 of TTVJ's price and threw in free shipping.
The thing is that it does actually prevent lowballing, and most importantly, it kind of 'finalises' the sale and transfer of ownership. The buyer and can now return to the FS forums and sell at any price he wants, without fear of some dude digging up the thread the seller bought from and saying "Hey but you only paid X dollars, how come now you want X+100 dollars!".
This is one of those frustrating head-fi oddities that never-the-less is a part of what makes it such a great place to be.