It is likely someone has asked you the 'question' in the title many times, in real life or in RuneScape. It can be considered part of a special class of requests (with "Does this make me look fat?") that have an obvious and truthful, but perhaps impolite, answer - hence the quotes. This psychological barrier of sorts is also subjugated to a ruthless and incessant stream of exploitation. It manifests itself in the form of a desire to scam items masquerading as insincerity. In other words, it seems people are constantly badgering each other to trust them with their stuff.
One seasoned scam is a promise by a player to double the money of another player. In principle, such doubling could be achieved by having the player show their money in the trade screen and subsequently handing over an equal amount. However (disregarding those players who claim to have a hack or other exploit), experience will affirm that this is not the method pursued by most of these so-called "doublers." Instead, the doubler will promise that once you hand over the money, an amount twice its size will be returned immediately, justified by a desire to see that you trust them.
Human nature and culture much of the time dictate that, given no prior experience, people want to trust others, apparently even after an encounter as brief as a ten second conversation on a video game. As absurd as it may seen, one can find many people claiming to have been scammed by various trust schemes, of which doubling is only one. Other, more elaborate schemes include 'trust games,' during which a scammer promises to give an item of substantial value to whoever lets the player temporarily hold the most valuable item.
On the other hand, although experimentation is not recommended, not all of the attempts to garner trust circulating around RuneScape have the ulterior motive of a scam. Personally, many friends asked me for loans, which I gave since they were token amounts to me. Of course, some were never returned, but others made a sincere effort and eventually returned my money. And judging from friends, especially those in high level P2P circles, some players lend items worth hundreds of millions, whether to help someone get back on their feet from a bad merch or to enable them to use expensive equipment to help a team in a PvP-type game. The claim is that such trading occurs within a group of people.
An interesting question is exactly how much trust is generated by sharing such virtual property. Many players will insist (and in my opinion, rightly so) that they have invested a substantial amount of time and effort to obtain items. The reality is that, strictly speaking, that any value possessed by these items is purely sentimental. After all, you can't swap, trade, or sell these possessions for anything but other items in the same domain.
Perhaps part of the answer lies, as usual, in connection with real life. Perhaps without realizing it, there is plenty of trust to go around even towards random strangers. It is quite possible that the last time you crossed a street, a car stopped and waved you ahead. And presumably, you proceeded to cross without fear that the driver would villainously floor the gas pedal before you finished crossing, whether out of intent or carelessness. (However, it would explain the glint in the driver's eye.)
Trust in RuneScape is a somewhat absurd commodity. Sometimes, it is given out too easily, as one usually sees from "doublers" and hosts of trust games. At others, it is perhaps withheld too tightly, and not necessarily always over money. Reasons and motivations for trusting or not trusting still remain somewhat unclear. My only advice would be that only you can decide for yourself who you want to trust over RuneScape, even if it means no one, trust me.