If your buying out of local dealers ALWAYS drive the car where possible before paying. If they don't like it tough, it's your money. As fair few of these cars will have been p/ex then parked up. The average salesperson won't know or care much about it, it maybe fine but it may also have a host of faults which a simple test drive will show. The salesperson hasn't paid a penny of his own money for that car and is only interested in balancing the books on the deal.
It's a little different if you buy out of Dealer-Auction but if you find your buying the same dealer a few time hopefully you can build up a relationship with them. If you use Sytner Auction it was in there terms and conditions that your allowed to test drive the car, point this out and use it where possible. Remember its your money.
In the price range your buying you will always get a few bad ones, it's happened to us all and still happens no matter how much experience we have. How you deal with problem cars can define your business. Sometimes it's best to get rid and move on. If its a sold car be fair with the customer and they will hopefully be fair with you.