Used car salesmen have long had a reputation for lying about the poor condition of the used cars they sold. A joke soon developed in the computer industry:
 
Q: What’s the difference between a used car salesman and a computer salesman?
A: The used car salesman knows when he’s lying.
 
The joke has been cited in print since at least 1985. “Software salesman” has replaced “computer salesman” in the joke since at least 1995.
 
“Woud you buy a used car from this man?” is another popular saying about used car salesmen.
 
   
22 July 1985, The Post (Frederick, MD), “Reporter’s Notebook” by Douglas Tallman,pg. A-7, col. 3:
All this reminds me of a joke floating around computer circles: what’s the difference between a used car salesman and a computer salesman? A used car salesman knows when he’s lying.
 
Google Books
11 February 1986, PC Magazine, pg. 56, col. 3:
A friend from Texas passed along the second-funniest PC joke we’ve ever heard, surpassed only by the classic Apple knock-knock joke:
 
Q: What’s the difference between a used-car salesman and a computer salesman?
A: The used-car salesman knows when he’s lying.
 
Google Books
4 August 1986, Network World, “Wanted ” Better PBX proposals” by Ian Angus, pg. 29, col. 3:
Question: What’s the difference between a used car salesman and a private branch exchange salesman?
 
Answer: The used car salesman knows he’s lying.
 
Cynical? Yes.
 
True? Too often.
     
Google Books
Adweek
Volume 29, Issues 1-11
1988
Pg. 26:
As Mike Antebi, a former art director and now a New York-based microcomputer consultant, jokingly cautions: “You know the difference between a computer salesman and a used-car salesman? The used-car salesman knows when he’s lying.”
 
Google Books
Using Microsoft Works:
Macintosh Version
By Ron Mansfield and Barrie A. Sosinsky
Carmel, IN: Que Corp.
1989
Pg. 86:
It occurred to me yesterday that the only difference between a computer salesman and a used-car salesman is that a used car salesman knows when he’s lying to you.
   
Google Books
How to Make Your Shrinking Salary Support You in Style for the Rest of Your Life
By Michael K. Evans
New York, NY: Random House
1991
Pg. 110:
What differentiates a banker from a used-car salesman?
The used-car salesman knows when he’s lying.
   
Google Books
AS/400 Security in a Client/Server Environment
By Joseph S. Park
New York, NY: Wiley
1995
Pg. 119:
Remember the old joke that asks the difference between a used car salesman and a software salesman? The answer: The used car salesman knows when he’s lying.
 
Google Books
A Rocket Scientist’s Guide to M.E.
By Richard Newlands
Lulu Publishing (Lulu.com)
2015
Pg. ?:
“What’s the difference between a used car salesman and a software salesmen?
The used car salesman knows when he’s lying.”
   
Forbes.com
BUSINESS 8/12/2015 @ 10:41AM
No More “Used Car Salesman” Jokes? Welcome to the New World of Used Car Sales
Celia Brown
There’s an old joke that goes “What’s the difference between a used car salesman and [insert any type of salesman, consultant, agent or other professional ‘persuader.’?  A. The used car salesman knows when he’s lying.”