Finbar Hannaford, 22, of Pembrey, Carmarthenshire, sold more than 300 belts using names of Italian and Japanese brands such as Sabelt and Takata but they were made in China.
Swansea Crown Court heard they failed at speeds of 50mph (80 kmh) and over.
Hannaford was given a suspended 18-month sentence after admitting fraud.
Carmarthenshire trading standards officer Lindy Evans said emails have been sent to more than 300 people who bought seat belts online off Hannaford between August 2014 and July 2015.
But she fears dozens are still wearing them.
The court heard Hannaford sold them on eBay and Facebook along with other counterfeit car accessories to be used for rallying, circuit racing and for "boy racers in suped-up vehicles".
Prosecutor Lee Reynolds said they had been tested, adding: "The products gave the impression of being Italian-made but had in fact been imported directly from China.
"The harness failed on several points indicating that in a real life situation - and under normal use - it could cause death or serious injury."
Defending Paul Hopson said Hannaford thought they were genuine.
Recorder Christopher Slee suspended his sentence for two years and ordered the defendant to undertake 150 hours community work and pay Â£1,000 costs.
He said the crash tests showed "how dangerous they were and one of the buyers could have been killed".
Semi-professional racing drivers could be in danger after buying "dangerous fake" seatbelts which fail at fast speeds, a court heard.