Ive experienced this first hand...
The place I worked previously was a main road 70 car used site. Back 6-7 years ago when it was busy it was selling 100-120 per month. When I left in July last year it was stocking the same amount of cars but struggling to 25-30 a month.
Now working for a smaller site on the corner of a business park 'out in the sticks' stocking 40 and doing 30 a month easily with internet and satnav being the AutoTrader magazine of the past.
Times have definitely changed!