Good question...firstly I should point out that there are no traders "like me"
I get cars from a variety of sources including Auctions, private advertisements, and from other garages who I have built up relationships with over the years. For example if someone were to trade in a 350Z ( or any other Nissan ) at a local VW garage, they would phone me for a bid on the car and and it would come to me when they got the car in.
I also get invited to the Nissan closed sales, which are a little expensive but we get good quality cars through them. I am in Scotland fornightly for the Nissan Finance sales which consist primarily of lease cars aged 1 to 3 years. These cars have been on contract hire with maintenance included, so are generally a very good buy but you pay good money for them.
And yes, I do search Auto Trader on a daily basis looking for private sellers who have undervalued their cars, or need a quick sale. In fact the 350Z I just sold came from just such an advert. I saw it advertised for a couple of weeks and decided to phone the guy. He wasn't having much luck selling it so I kindly explained that most people spending £20k for a car want finance, have a part exchange and they want the back up of buying from a reputable dealer...and made him an offer. He phoned back a couple of hours later and accepted, so I went to get it.
I don't just buy Nissan. I stock about 30 Nissans on average, but I also keep about 30 other cars to offer some variety.
Dan, is your garage in Rossendale? I'm just down the road at the Bury junction. Always interested in finding decent garage sales for when I want my roadster.....