Neither bothers me. In fact I see both as an opportunity.To summerise, I've bought hundreds of cars, and I'm put right off when an advert says "relisted due to time waster"
And I'm put right off if a seller wants a £500 up front payment "non refundable" when I haven't seen the car.
If someone has been let down by a buyer they may be more keen to sell having had their time wasted. Perhaps a discount awaits for a quick sale.
And a £500 deposit clearly puts at least you off and probably others. So less people for me to compete with
I also tend to associate those who lay down stipulations like that as being business like. Though clearly you need your eyes open to fraudsters.
Now whether I'd actually part with £500 before seeing the car is a different matter. Like I say, I'd rather travel and do business face to face there and then.