Translation: 'I have no effing idea what's wrong with it but I put this in the ad to make it seem as if I do. I do know it will be expensive as hell to fix so I'm trying to dump it.'
So offer 1/4 what they are asking if it's something you wouldn't mind fixing up. If they get offended, point out nicely that you are rolling the dice.
Sometimes they won't bend at all; I looked at a FB once where the guy said 'starts and runs fine', IIRC he wanted $900.00 for it. When I got there, the plugs were out and had been for quite some time, there were several different types some not for a rotary laying around the engine compartment, the carb stank of very old gas. The 'eyebrows' on the rotor housings which prevent installation of the wrong plugs had been ground off, all the alarm bells in my head were ringing. He wouldn't budge from the price even though I could not hear it run. I then said 'okay, get it running and I'll come back to see it'. His response? 'Then the price will go up'. Really? 
He then said 'trust me it runs' and when I pointed out that as it sat there was no way I could confirm that it did he got all offended that I would suggest it didn't run. I said 'I trust you but this engine is another matter'. I thought he was going to take a swing at me so I left.
And be done SOONER then what that job pays. 
