Fundamentally, haggling is a tug-of-war battle between two people with imperfect information. As the seller, you do not know how much the buyer is willing to pay - as the buyer, you do not know how low the seller is willing to go. Even if the buyer/seller tells you exactly how much they are willing to accept, there is no real way for you to know if they are telling the truth or whether it is merely a gambit on their part. In fact, it's entirely possible that the buyer/seller does not even know the truth of their own statement.
Within World of Warcraft however, there is one piece of information you do know when it comes to people selling items through Trade Chat: they are desperate.
"But... Azuriel! I/my friend/some dude sells things over Trade all the time and isn't desperate at all!"
"You/your friend/that guy is merely a good haggler and/or enjoys it."
Think about the market for your item, say, a Volcano deck. Who would buy it? You might have raiders, you might have PvPers, you might have alts with sugar-daddy mains, you might have just some random guy just hitting the level cap. Your market would include people specifically looking for that deck, and it could also potentially include people who were not specifically looking for the deck, but just happened to stumble upon it and thought it would be good for them. Depending on the price you ask for the deck, you might also create demand for it from goblins willing to assume the risk of flipping it.
Who is excluded from your market? People whom have been priced out of the demand curve of your deck. You also exclude people who never knew you were selling the deck to begin with.
All of the above assumes you listed the card in the AH. Why? Because by selling it solely in Trade chat, you exclude another group of potential buyers: everyone not currently in Trade chat. Think about that. You could have raiders and PvPers fawning over your item and willing to pay through the teeth... but they are currently in a raid/Arena, and not around to even know you are selling. Or, hell, they could merely be out doing dailies or questing on an alt. Or AFK.
So your entire market is solely the people currently in Trade chat. No reasonable person would pay more for an item from Trade that he/she could buy from the AH for less. In fact, everyone instinctively accepts that AH prices have a "premium" built-in for the convenience of being able to buy at one's leisure, which means they instinctively know the opposite to be true, e.g. things are cheaper in Trade chat. Any seller that consciously chooses to handcuff themselves by selling in Trade, almost by definition, is either desperate or willing to limit their profits to push product now rather than later. I would call the latter "desperation" anyway.
I typed all the conceptual framework above as a preface to this otherwise simplistic suggestion: low-ball the hell out of them. Keep it in the realm of possibility, but low-ball it.
Let me give you a literally true example that occurred just two days after my Volcano hagglee experience, and is the height of irony. For one thing, I was on my Inscription toon reposting unsold Darkmoon decks when, lo and behold, someone came on Trade wanting to sell a Volcano deck for 16,000g. I whispered them with an offer for 12,000g. Why 12k? What I wanted to do was provide a tantalizing floor at which someone might end up going "screw it" and selling it at. If I had made a truly absurd offer like 8000g or less, they would dismiss it out of hand as a troll, just like anyone in Trade chat offering 1g for it. Personally, 12k is pretty ridiculous by itself for a Darkmoon deck worth 16,000g in Inferno Inks, but if someone wants to pay the reverse-premium of a guaranteed sale right now, let'em.
I did not actually get a response right away from the seller. I saw him barking it up a few more times, and I whispered "I'm still willing to buy at 12,000g." He came back with a "Well, 12k is kind of low. I'm splitting it with another guy." Remember when I said I was a big softie? Yeah, well, it comes and goes. There is no way for me to know whether he is serious or just cynically playing the heart-strings, so my default bargaining state of not believing anything they say kicked in. "I'll buy at 12,000g if you end up changing your mind." This is my signature haggling move, the Artful Disengage, in which I indicate "this is my deal, take it or leave it" without actually achieving that aggressive posture. Sure enough, the guy went back to Trade with "WTS Volcano deck, have 12k offer. If you want it, offer has to be 13k." Interesting gambit on this guy's part and it almost inclined me to offer 13k just to seal the deal.
Remember from last time when I mentioned that my internal bargaining position would be weaker if I was selling something I could not actually use/have a use for? It also works in reverse: I already had an unsold Volcano deck, so it was not as though I was emotionally invested in this transaction outside of the gold-making possibilities. I use the Artful Disengage no matter my level of personal involvement, but this time I actually meant the "take it or leave it." After another 5-10 minutes of waiting, the seller took it. I met him outside the Dwarven District AH, traded the 12,000g, and then I immediately walked 15 yards to the auctioneer and listed the deck for 21,000g, the same price as my other Volcano deck.
After logging on the next day, I discovered someone had bought one of the decks for 21,000g. That is a profit margin of over 8,000g. You can be damn sure that the buyer was NOT someone in Trade chat the night before.
- When you see someone selling on Trade, try to establish a floor for them. The "floor" in this case is a bid high enough to make them think twice about *not* selling it to anyone, while low enough for you to have room to maneuver should they drive a harder bargain. If you read my Hagglee post, you should now see why I had you establish a floor for yourself, lest someone else try to do it for you.
- If you find yourself bad at haggling, you will ironically be more successful the less important the transaction actually is to you.
- Usually goes without saying, but being aware of the market price for the item in question is important, especially when the seller is giving away no information (other than their inherent desperation), such as when they just say "WTS X, pst" or "make an offer." The seller is secretly hoping that you, the buyer, will make an offer higher than their expectations. If the seller wants 50g for the item, they hope you offer 60g and think you are getting a bargain.