Sometimes sellers flake if they get a lot of requests. "If someone else wants it, then maybe I should too!"
I think this is what happens sometimes, but not always. I've had a LOOOOOT of sellers flake on me. The '86 Z28 I bought for parts took an eternity for the seller to actually get back to me, I had to hound his phone for three weeks until one day he answered and told me he hadn't received any other phone calls.
I also know of a local '70s Suzuki 4x4 hiding in a friend's sister's ex-husband's garage, been parked there since the mid '80s and buried in shit. Once a year the guy gets interested in making it go away, but as soon as anyone shows interest, he clams up and decides to keep it.
He almost donated it to auction twice already until someone else offered him money. WTF. He doesn't want it if it has no value, but if someone else wants it, he does.
It scares the bejesus out of me that someone, somewhere ordered the Lusitania with front drum brakes.