The common misconception is we make 20-30% margin on car which simply isn't the case. So buyers thinking they are going to haggle £2k off a £15k car because they have cash and no p/x are going to be disappointed.
Lets take an example, 2008 CLS priced at £15k.
Cost to buy vehicle £12500
Fees and transport costs £350
Prep cost (valet, mot, wheel refurbishing) £200
Tyres/servicing (on average) £300
Warranty £180
Total cost £13530
So if that car sells for the full asking price the profit remaining is £1470.
Vat margin on the gross profit is £500 (less the vat paid on any parts, a lot of valeters, wheel refurbers are not vat registered)
So we make £1030, less advertising costs which for us are around £25 per car per week.
So that's £1030 profit left assuming you get the full asking price, and the car had no major faults. The likelyhood is that you won't get the full asking price and 1 in 3 cars need more prep than you had estimated.
So all in, less than £1000 profit on a £15000 car. Thats before any fixed overheads, so there isnt bags of money to be made from selling used cars.