Best one we had were a mother and daughter who came to look at a 10 year old fiesta we were selling for a friend (we have parking for more than one car - he doesn't). It was cold so my husband parked it in the garage. They looked at the car for a bit and seemed quite impressed so he went in to make some tea. When he came back they'd left the engine running with the doors to the garage pulled closed and were concerned that it was 'running a bit fumey.'
Really.
When I was selling my very first car, advertised on eBay (a 98 1.3 Fiesta Finesse), I had it up for maybe £1,100 or so, hoping I'd get over a grand for it, maybe.
The only query I got from a watcher wasn't a query at all - instead, it was a "lol, u'll nvr get that 4 it m8"-type comment.
I went to see what he was selling, and amusingly it was such a shonky Mk3 Golf you'd struggle to shift it for half the £800 he had it up for.
While I didn't sell the Fiesta through ebay in the end, someone at my dad's work paid the full £1,100 for it. Result. Also, in-your-face to the dummy with the crap Golf. Bet it took him ages to shift...
I had every one of those when I was selling my Racing Puma. Ebay is usually the worst for it too - I even had an offer £2500 less than the asking price, and numerous 'txt spk' enquiries.
I sometimes think it's easier to just part exchange and take the hit, rather than dealing with some of these idiots.