As long as there is an economy, people will try to cheat others out of their money to make a few quick GP without doing any actual work. This is why every MMO, including Runescape, has rules against this sort of behaviour. Of course these rules are rarely a deterrent, and there will always be people that think they can beat the system. As new scams become popular, Jagex takes the appropriate measures to deal with them. Some of these anti-scam measures have ended up being a part of the daily Runescape experience.
An early example of this is the unique look of Dragon items, compared to similar items made of other metals. When the dragon longsword and battleaxe were first introduced, their inventory sprites were, like all other metal items at that point, simple palette swaps of older sprites. This was exploited by people who would offer a sword or axe for trade, then trade the dark red coloured dragon item for a brown coloured bronze item at the very last moment. To put an end to this the weapons were given distinct sprites that made it easy to tell the difference between dragon and bronze. Of course graphical limitations still meant that dragon items still looked like dark red versions of normal items, but when RS2 came around their ingame appearance was given a more distinct look.
Another Runescape Classic classic abused the fact that the width of an item slot allows for stacks of up to seven digits before the number starts flowing over into the next slot. This was exploited by a few 'clever' players who used a combination of two stackable items: coins and needles. They would, for example, offer ten million GP for a party hat, then offer one million GP and a single needle. Because the inventory sprite of the needle was very thin and because the trade window was partially transparant, the needle would be virtually invisible and it would look like the player was offering 10.000.001 GP. To put an end to this Jagex implemented the trade confirmation screen that would clearly list the items being transferred, thus exposing the fact that you were about to receive one million GP and a needle in stead of the expected ten million.
Something that persisted well into RS2, even with the presence of the confirmation screen, was players with quick fingers swapping items at the last second for something cheaper that was very similar both in appearance and in text in the confirmation screen. For example, 100k GP would be swapped for 1004 GP, yew logs would become regular logs at the last second and silver ore was replaced by tin ore. The first measure taken against this was adding colour to the number indicating the size of the stack (white for anything up to 99.999, cyan for anything up to 9.999.999 and green for anything higher than that). Quick swaps became all but impossible to do when Jagex added an angrily flashing red exclamation point to the trade screen that would appear whenever a change was made, and was truly stamped out once and for all with the implementation of balanced trades.
For as long as the game has been around people have been offering their skills to help others process raw materials. Doing this would give them the XP reward and their 'client' the finished product. This was most commonly done with gemstones to raise the crafting skill, but was also done with comparable skills, such as smithing and crafting. However, there was an inherent danger to this scheme as there was nothing to stop the other player from simply walking away with the raw materials. An interesting side note regarding gems is that since the introduction of the Grand Exchange the price of uncut gems compared to their cut counterparts has risen considerably. Because of this Jagex was forced to change the examine text of uncut gems, which had been the same since the start of the game, from the classic "This would be worth more cut." to the simple "An uncut X".
All this was ended with the introduction of the Assist System on November fifth, 2007. This allowed players to 'borrow' the skills of another player and produce whatever goods they needed (with a few exceptions, such as raw materials, quest items and items that have a quest requirement). In return, the assisting player would receive all XP. This meant that players could now turn their raw materials into finished products without the items ever leaving their inventory.
Related to the previous scam are the herb scams. For a very long time all herb drops from members would simply be an unidentified herb, with no way of telling what it actually was unless you had a herblore level high enough to identify it. These levels were similar to the levels needed to clean herbs today. Players would commonly identify whatever herbs they could with their herblore level, then offer everything they could not use for trade, commonly using phrases such as: "Selling unidentified herbs, avantoe and up.". Players would know that Avantoe requires level 48 herblore to identify and thus that the herbs they'd be buying would be relatively high level herbs such as torstol, lantadyme and dwarf weed. The problem here was that each unidentified herb still registered as a separate item, and thus would get its own bank slot. This meant that a player could simply take one herb from each stack and identify it, thus being able to offer a whole stack of unidentified guam leaves and marrentills while passing them off as potentially being more than they really were. Of course, it was also common for people to offer to identify herbs, which, like the aforementioned gems, had the risk of the other player simply walking away with them.
To end the herb scams and to make trading herbs easier, unidentified herbs were replaced with grimy herbs on September tenth, 2007 as part of a larger update to the herblore skills. Players would now know what kind of herb they were holding from the moment it was dropped, and players could no longer pass off unidentified herbs offering false potential.
There are of course many more major and minor scams which affected the game in smaller ways after being fixed. Jagex staff impersonation has led to a small gold crown being added in front of all usernames used by Jagex staff. The scam of getting a player to drop an item in your house, then quickly expelling them, has led to a warning message being shown whenever you try to drop something in another player's house. As players continue to find new ways to scam, new fixes will be introduced to combat those scams. This might sound trivial, but like in Runescape, the real world is filled with things that were, directly or indirectly, implemented to protect people and help enforce the rules. Everything from door locks to zebra crossings falls under this category. While comparable measures in Runescape aren't quite as drastic, they are still quite present at times.