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Because some traders often spam trade messages in the local chat channel, ArenaNet implemented a chat filter which will automatically redirect any message containing "WTS" from the local chat channel into the trade channel. See Game updates/20060523 for examples on how the spam filter works. Additionally, all messages containing WTS in both local and trade channels is added to the Party Search panel and displayed as "message (Party Search)"
Some people try to get around this by changing the abbreviation slightly, such as "W.T.S.". If these people are annoying you remember to put them on your Ignore list, or close the local channel while trading. Sellers often bypass the filter in order to attract more attention to prospective buyers without triggering the Party Search message, as well as being able to advertise more items on-screen as opposed to the limited screen space on the panel.