Natalie James bought a car from Go Vauxhall in Croydon, south London, last year, but complained at the time about the conduct of salesman Gary Brown, who sold her the car.
Her complaint meant that the 37-year-old salesman did not receive the £200 commission he would normally pocket for selling the vehicle.
Eight months later, she and forgotten about the incident - until she received a furious Facebook message from the salesman.
It read: "Thought I recognised you. The complaining little slag!"
A confused Natalie replied: "Excuse me? I don't even know you?"
However the incensed car salesman, who had spotted Natalie on a mutual friend's Facebook page, added: "Oh yes you do.
"It was me that sold you the black Corsa, but then you complained about me for no reason at all and said that you didn't want to speak to me.
"I ended up losing commission I had earned for selling the car. All because you are a moaning little c***."
Natalie posted a screen grab of the shocking messages on Twitter, writing: "I bought a car from Croydon @vauxhall last year.
"Today I get this in my inbox from salesman who served me. Disgusting."
Police are currently investigating the matter, but Gary Brown has since issued an apology via Twitter, saying that he only messaged the woman because she had "hurt" him.
He said: "I saw you on a friend's Facebook and instantly remembered who you were when I sold you a car last year.
"I was gutted when you asked to speak to another salesman and had no idea what I had done to upset you so much.
He added: "Obviously that doesn't excuse my message yesterday, so I apologise for that."
The former car salesman also revealed that he had left the industry, as he did not believe he was suited to the career.