I believe it is a combination of what Smoot and Money Bags McGee said. This may be a bit of a detailed explanation, but if you can bear with it, it may provide you with an answer.
It can be hard to accurately determine the true market price, for reasons I will not get into. Because of this, placing a reserve too high or too low can lead to one party being made substantially better off than the other, which sure its nice to get a "deal", but I believe in fair trading, which occurs at the true market price as McGee said before.
With all this being said, it can actually be better (most fair) to not set a reserve, and allow consumer's to reveal their true preferences. This method can avoid pricing people out of the market, and theoretically, if demand exists, should drive up any low offers someone might make until the equilibrium price is reached. However, a problem exists in that, these forums target only us who read them, and not the population as a whole. Because of this, the final selling price could be either an overstatement or understatement of what the items true value is. We know this happens, but accept it anyway because usually the final selling price reaches somewhere close to the true value. However in extreme cases, the final offer may be so grossly over or under stated that we inherently know it is unfair. In a real market, this is when no deal would be made, and this is a right that both the seller and buyer have as participants.
So finally what we have is a problem of incomplete information, and a small problem of integrity. The seller doesn't want to make an unfair reserve, but also doesn't want to be made unfair offers. Everyone has the option to refuse trade, regardless of a reserve or not. What we instead hope for, is that members of this community will make only fair offers (close to true value) when no reserve is present. If we all look out for each other, we will all be made better off. In summation, if we all stopped making selfish (low ball) offers, then more trades would happen, and we wouldn't be having this discussion. Don't blame the seller, blame our selfish nature and what capitalism has taught us to do...