From a punter's point of view- there are good and bad garages too remember. I think that often, this nit-picking is their way of sounding you out, and also, some really haven't got a clue what to say or are nervous etc. so come out with random nonsense.
Today's messer can be tomorrow's buyer (or next year's buyer).
Just smile politely and move on is best I find if they're clearly not managing their expectations. As has been said: you do't 'have' to sell them the car as much as they don't 'have' to buy & it's always worth qualifying them to see if it's worth your while. I've found that if they're a pain before buying, then that's nothing to what they'll be like once they're in it!
Most reading this will know that you'll work much harder to satisfy a buyer at the lower end of the price spectrum than at the mid-range (as it's likely to be all their money).
We find that the Audi and VW buyers as well as small car buyers are often v. hard work too; so I adjust our stock profile accordingly where possible.
P.S.
We've tried a few WAV cars and yes, nice profit but have always taken eons to sell.