Most of the time, I don't do anything.
But sometimes, if I am bored and I have time and if it is something that intrigues me, I will play along, even if I have no interest in buying it.
I ask for more pics, what condition the game is in, does it even work, (a lot of times these morons post very little if any info about the game's condition), ask what other games they got, try to get a dialoge going... what is the history of the game, try to find out how they arived at the asking price.
I usually end up politely declining to buy anything, as I can tell they won't come down off their price enough to make it worth my while.
For example, one time I saw this post for a project cocktail for $350 with working monitor, no board, new JAMMA harness and controls. So the poster and I email back and forth. I ask him for pics, turns out it is a beat up Nintendo or Taito style cocktail, the monitor is a 17" computer monitor he just dropped in there, the back of the monitor is sticking out the bottom of the cocktail... and keep emailing him, turns out the joker is 3 hours away... he offers to meet me half way.
I end up telling him lookie here home-skillet, I have been in this hobby for years, and I know what the parts are worth, any you don't have $350 worth in parts there, you got less than $100 on a good day, and I am not driving 1.5 hours to meet you and give you money for that when I have 4 cocktail projects in my garage already that demand my attention...
Needless to say, I did not hear back from the guy after that.
On another occasion, I saw an Asteroids Deluxe for $4000. Now I was looking for one at the time, so I email the poster. She says it was her husband's who bought it from some specialty antiques shop in California and had it shipped to Georgia. They paid $5000 plus shipping for it so she wanted $4000 firm now that her husband had passed away and she did not want to keep it.
Anyway, I send my condolences, along with some links of BIN ebay auctions to show her that the market price for an AD is nowhere near what she is asking, and I even send her a link to an AD on the Chattanooga Pinball website, which list for less than half of what she is asking. I even told her that KLOV member Jim from Newnan had a completely restored AD, new sideart I am talking a perfect example, and he was only asking like $1000 for his.
Anyway, I tell her my best offer would be $650 sight unseen, which was generous compared to the fair market value at the time. I told her I know most of the collectors around the area and they would not give her even that much for it.
Anyway a few months pass by, and I get another email from her, she now wants $1500 for hers. I tell her my offer still stands at $650, even though I had already acquired one for $400, I would still buy hers for $650 because hers was in better condition. Haven't heard from her since...