Scamming is a term that is widely used for a lot of different things. Its most popular meaning is "Taking items, meat, or other things from people in any unfair way". There are a lot of different scams. Here is a list to some common scams. Most can be prevented by simply watching out what you're doing.
"The trade system is not working"
In this scam attempt, the scammer arranges a trade with the scammee, then claims that their trade system no longer works and asks for the scammee to send them the meat/items through kmail instead, then runs off with them.
The comment box trick
The "comment box trick" uses the note feature of the trading system to make it look like they have included the meat, when they actually have not. This relies on the trader not paying close enough attention to the trade.
While there are some bona-fide raffles in the Kingdom, scammers often exploit the trust of rafflees, tricking them into sending meat for a raffle which never takes place. This is commonly found in mall stores - buy the 5000 meat meat paste for a chance of winning the raffle - but can be engineered through other means.
Price-switching is a scam in which the price of the item is changed between searching and purchase in The Mall.
Often, the scammer will annouce a sale of an item at a very low price in a channel, such as trade, and then wait for players to start buying the advertised item. At this point, the scammer will raise the price on the item to a very high amount, sometimes catching an unwary individual who happens to have a loaded wallet, and effectively stealing a large quantity of meat from him or her.
This scam can be avoided by being sure of the price of the item you are buying before hitting the 'buy' button - if the item's price changes while you are browsing the store's item list, the purchase will be aborted.
Clan looting is taking items out of a Clan Stash for personal profit while having permission to take them. Jick has put the Karma system into place in order to empower clan administrators to moderate the value of items people can withdraw, but clans who manage their stash poorly or give out too much Karma to new members are susceptable to scamming in this manner.