It's because of this that I take a hard line. I use ALL of mine, including p/x shite, for a good few miles & prove it via the photos to potential customers.
One thing I've learnt is don't fear the courts unless you are actually going. 99% do not have any intention, or the ability to complete the forms, to instigate proceedings. It's all bluster & bullshit from people running their lives by what they hear from their ignorant mates & read on the internet posted by other whingers.
You're absolutely correct about people expecting you to subsidise their lifestyle with their secondhand cars. Times are frigging hard but £1 only buys £1 of goods, no matter how hard times are. Be fair, but be hard & don't be afraid to make it absolutely clear that you don't take shit.
I attempted to educate a punter & his Mrs the other week when looking at an 09 Zafira for £2K and complaining about the dimple on one door (it was an absolute disaster to them, Champagne taste, lemonade money) "Sir, the fact you've scrimped & saved £2000 counts for nothing. Whether you piss £2000 up the wall per night in the casino or you've sold one of your kids kidneys for the £2000, £2000 buys you £2000 of goods". I have to say they appeared somewhat surprised.
Anyhow they've gone for a coffee to discuss it & said they'd phone me back. It must be a big cup cos it was a fortnight ago & they've still not phoned. I wonder how they keep the cup warm for that long???