Why would he need to?
Because thats pretty much how much they are worth.
The first 1.8 litre Vtec honda I put in to autotrader comes up with a 2006 car, with 50,000 less miles and £20 cheaper.
£500 more buys him a car with 70,000 miles.
£600 buys him a car with 24,000 miles, all 2006 cars.
A 24k mile Civic on an 06 plate for £5500? You can probably hear it barking from here given TRADE book on that car is £6500. You could bang it through auction and get loads more than £5.5k on an 06 with 24k miles on it so if you dont mind I'll pass on the chance to buy a 'low mileage' example for at least £2k under what you'd expect it to be advertised at!