This is a difficult one and will depend (in my view) to a large extent on the seller’s circumstances.. If they need the cash for deposit on next car, pay off finance, etc. and they’ve had zero interest so far they may take a hit. If they don’t need the money and the car is a good spec (right engine/gearbox combo, colour, options, SH, etc.) then they may just stick.
I was selling a 325 a couple years ago and had little interest at 8K and it was getting close to the point where I needed the deposit for my new car.. Someone came to look at it after 3 weeks on Autotrader and offered me 7K.. I think we landed on about £7.6K (it was a good spec/history etc. and although I needed the money I wasn’t willing to go too low).
If it’s local and you’re able to go and have a look it helps massively as you’re not beginning the negotiation process over the phone without having seen the car.