PST is the acronym for Please Send Tell. It is not a sound effect. It means that you would like somone to send you a Tell - a private message in game. This is not always called a Tell by all games, sometimes it is a Whisper, somtimes it is called other things, but the meaning is the same.
Usage: WTB (Want To Buy) sword of holy crap PST
Incorrect Usage: WTB rod of doh! PST to me - If you are asking someone to PST, it's automatically assumed that they are sending the Tell to you.
WTB holy ghost please PST - You just typed: please Please Send Tell. Kind of redundant, don't you think?
WTB fake poo PST me with a private message - Again, it's not a sound effect.