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Here are three example charts someone asked me to read and interpret from Guild Wars 2 Spidy. You can turn off all but the sell listings/buy listings volume and sell/buy listings raw data by clicking on the other keys in the legend for each chart. Those will then get grayed out and only the four listings I just mentioned should remain visible.
#1: Great Market
#2: Good Market
Another great market! You can see that whenever the buy listings outnumber sell listings, then prices go up on the listings.
#3: Terrible Market
Very strangely profitable market. I notice that there are many people trying to buy and very few trying to sell. My guess is that this is something like a recipe that doesn't have a lot of items actually on the marketplace. Probably not a consistent market to play! It sure looks good, but looks are deceiving. You're up against tons of buy orders competing with yours and not enough actually selling. You're just going to get over-cut and be lucky to even see one of these items. Selling would be a piece of cake though.
I hope this analysis helped!