I usually think of used car salesmen as being sleazy

In the 80s, in Oz, there was a comedy TV show (quite ahead of it's time, IMHO) called "Australia: You're standing in it" which featured a couple of Used Car Salesmen Wayne and Arthur Dodgy, henceforth known as the Dodgy Brothers. Their image, and the sketches they used to do, is probably the reason most Australians would think of UCSpeople as "dodgy".

In fact, after googling this, I've found that many Australian sites use the term "dodgy brothers" for any business which is very low standard eg. "the least said about the previous site created by some Dodgy Bros. Inc, the better". Pop culture strikes again, huh?

http://www.nostalgiacentral.com/tv/australiastand.htm reference at bottom of page