Basic Maths
£15000 / 12 = £1250 Profit per month
£1250 / 5 Cars per month ( 1 a week part time ) = £250 Profit per unit
However 60 cars a year at circa £2000 per car = £120,000 turnover so you'll need to VAT registered.
Autotrader 5 cars per month circa £400 a month
Gumtree another £100
So to take £250 per car after Vat/prep/Advertising from a £2000 car I think you'd be doing well
Also this is before - Screamers/Consumer Protection act/Lawsuits/Warranty claims/people turning up with a broken down car kicking your door in.... etc - of which there'll be plenty in that kind of market
Just my 10p worth.....