Allow me to explain the major differences between speculation and gambling on the Guild Wars 2 Trading Post. To the untrained eye, I am sure that speculation is the same as gambling. However, look at it through Markco colored glasses, like so:
A gambler puts his money down on an item, buys a bunch of it, and then leaves the purchased items in their bank until prices drastically rise and they can cash in. The only reason for buying is the chance for a big pay off at the end. If the items do not make gold then the gambler loses pretty much everything they invested. They often buy too late in the beginning and sell too late in the end.
A speculator looks at far more variables than just potential profit. They analyze whether there are ways to slowly get out of a bad situation, such as selling off their inventory should things look less than promising. A speculator also has alternative methods for selling off items, such as crafting, salvaging, etc. Basically, the difference between gambling and speculating is that you take action between the ordering and selling of the items. There's a lot more thinking and calculated risk taking because a speculator recognizes that there is more to economic gambling than just placing your bid and waiting for the pay off.
To put it simply, the speculator has more than one way "out" of a bad deal, and he or she doesn't expect to become filthy rich off any one "gamble."
Be careful with your speculation and avoid risky investments, especially if you can't afford to lose.
There's talk of Azurite speculating both on the forums and within the blog comments recently. Are you going to speculate or gamble on this information? I recommend it as a decent risk vs reward proposition. Definitely pick some up for cheap now before too many people invest at it's current low price!
(Oh and btw, the Embellished Brilliant Azurite Jewel is flippable for a nice profit and there aren't many on the trading post at the moment)