Todd Casey had recently swapped a vehicle with a Dunedin man, who then claimed the vehicle he had received had a faulty fuel pump - a claim Mr Casey rejected.
On Wednesday, he was seen by a group of people in a white van, one of whom was apparently the person who had swapped a car with Mr Casey and was demanding money.
The white van sped through the car park and stopped behind Mr Casey's vehicle, blocking his exit. A man then rushed out of the van and stabbed Mr Casey with a pocket knife.
He suffered a small cut in his arm.
As of yesterday evening the assailant was still at large.
Police yesterday warned people to exercise caution when buying items off social media buy, sell and swap pages
A police spokeswoman said in the past six months, Southern district police had received 23 reports of residents being swindled on such pages, the collective value of which was $16,741.
Police recommended using "reputable businesses such as Trade Me as they offer a greater level of protection, but if you are using a non-reputable site like Facebook then make sure to confirm their identity before completing the sale".
Fraudulent sellers often asked for money to be deposited into an account before the buyer spoke to them or could confirm their identity, the buyer then never receiving the item they paid for, police said.
"If you suspect a seller may not be legitimate, do not complete the purchase and report the matter to police.
"If an offer is too good to be true, it usually is."
Consumer NZ head of research Jessica Wilson said those buying from private sellers through a social media group were not covered by the Consumer Guarantees Act (CGA), which protects consumers who buy faulty goods or goods that never arrive.
"The Act only covers you if you're buying from a trader.
"Doing business with other individuals through these groups means you won't have any comeback under consumer law if something goes wrong with a purchase.
"These marketplaces are risky and there's much less protection for consumers when things go pear-shaped."
Ms Wilson said the man who stabbed Mr Casey was not entitled to any protection under the Act, as they had arranged the deal as private sellers through a social media group.
"If you end up with a car that has a faulty fuel pump, you won't have any rights to a remedy under the CGA."
