From what I can tell, this stems from ANet's lack of foresight to place correct monetary values on items that are exchanged for other items of equal (and many times lesser) value. Ever heard of Troll Tusks?
I buy Monastery Credits for 75-100g each, then exchange them for ale, which I then take to Droks, proceed to get drunk for an hour straight, and spam WTB Troll Tusks, and chances are, I barely break even, not to mention I still need to run the tusks to the trader. Now, if I were a 55er or W/Mo that had just been out farming, and I see a person wanting to buy my tusks, I have 2 options:
1) Sell em then and there, and get back to farming for more tusks faster
or
2) Hold onto them, and then spend the time later to run to the trader and exchange them and so on and so forth
Now, it comes down to convenience. Do you want to spend more time making more money, or less time making less money? Whether or not one is faster than the other is not something I want to test out, if it can even be done considering the variables involved.
Now, onto the Commendations. Sure, a good majority of people know how much commendations are worth, and some do not. However, there is more behind the person spamming WTB Commendations 150g etc.
1) How many people have actually sold to him? I have been in districts for 10 minutes at a time and have had no replies to buying and selling Black Dye, doesn't mean I have gotten or sold a dye for each time I spammed.
2) How many people just want the cash right on the spot? Honestly, I have merched greens a few times, simply because it isn't worth my time to sit around and try and get that 1k for Shreader's Talons, or some other less-sought after greens. And let's not mention the ever popular Shing Jea greens, those are always a joy to see dropped...
3) How many people do not know any better, but see that they are getting 150g from an item that merches for much less? In the end, it is their choice to sell those things, not the buyers' choice. It is always the seller's choice for the most part, and if the buyer makes clear that he has a firm price in mind, then it falls onto the seller, does he want the cash now or will he hold out for more?
Basic economics is at play: Buy/sell now at price X or buy/sell later at price Y, but the thing is, over time, which is more effective, buying/selling X and then continuing with anything else you had to do, ie make money, armor, quest, dyes, etc, or holding out for price Y and sitting on your cash/item for the perfect moment, which may take minutes, hours, weeks, etc. How do you want to spend your time to make the most of it?
During the Dragon Fest, there was a mda rush to get orbs during the opening hours of the event, before the quests. Turns out, people were buying orbs at ridiculous amounts of money. Now, ANet makes a few quests the next day that give out 240 orbs. How do the mad rush buyers feel now?
I can tell you that I made a good amount of cash by buying orbs at a low price and then selling them later for a higher price. Took a butt-load of time and effort on my part, but it worked. I didn't want a mask, nor did I feel like I was cheating people, because I was able to get orbs from willing sellers and sell to willing buyers. In the end, everyone won, because they wouldn't have made those choices at the time had they not wanted to. Sure, looking back, it seems like it may have been considered a possible scam, but then hindsight is 20/20, and orbs are worthless now. So it comes back to what do you want form the time you spend making/spending cash in game.
And there is a whole other can of worms to open about the Ranik sellers in Pre-Searing, talk about people being lazy....
My opinion: let people buy and/or sell at the prices they want, it will fall to the other people in the district to decide how they want to respond to the text that flies ever so rapidly during peak selling hours. And it also depends on whether the seller is determined enough to sit through any flak or lulls in trading that he may experience.