When a player is trading, the wealth of what they receive or give is affected by the trade limit which was introduced in January 2008.
The trade limit was released as a method of combating people scamming other players and preventing real world trading.
The trade limit means that if you were to buy an item then you must pay for it with around the exact number of coins that it costs on The Grand Exchange. How much higher or lower you can pay than the market value depends upon your number of quest points, with those who have low quest points being unable to trade too far off of market value whereas those with high quest points can pay many thousands of coins less or more than market value. The trade limit counter restarts itself every 15 minutes.
Quest points Limit (in coins) 0-10 5,000 34 10,000 75 18,750 150 34,615 225 50,480 270 60,000
When trading, both players have to be both giving and receiving within their own trade limits, so if you were able to have a trade limit of 10,000 coins but your trader was only able to trade 1,000, you would only be able to provide them with wealth within 1000 coins of what they have given you.
Although you might have a full limit allowed for your quest points, and the person you are trading to does not, their limit based on the quest points and how much they bought/sold still applies to you. An example is that if you have a limit of 10,000 coins and so does the person you are trading to, but they recently lost 3,000 coins. Their new limit is 7,000, and that limit applies to you also, not the 10,000 limit.
The trade limit affects all tradable items. It does not, however, apply during minigames such as Castle Wars. It does affect item lending though, and this can be a problem for those with an item lending business. If you want coins in return for lending out an item, you will not be able to charge someone with the market value of the product because you are only lending it, not trading it. Therefore, to take money using item lending, the person borrowing can give an amount of wealth within their trade limit to you as long as you can accept it to. Item lending would be treated in the same way as a one-way trade in which only one half of the trade provides items.