She was driving the car at speed on a busy Sydney road when it suddenly lost power, as if the fuel wasn't getting to the engine. "I went from 60 to zero in about 100 metres," she says.

The 32-year-old freelance marketer had bought the eight-year-old Renault Megane privately for $5000. She took it for a test drive and the seller said there was nothing wrong with it.

Meredith managed to restart the car after about 20 minutes. She took it to a mechanic who could not find anything wrong with it.

"That problem went away for a while, but there was a multitude of other problems," she says.

"I had four flat tyres in the first month, which I thought was very weird, and the fabric on the inside roof of the car fell down, which made me think that it had been stuck there with some PVA glue."

She spent about $1500 getting these things fixed, but then the engine problem reoccurred and kept getting worse.

She sold it to a mechanic who believed he could fix the problem and received a fraction of what she paid for it.

"I wouldn't call it a lemon, I'd call it a lemon tree," she says. "It was a nightmare."

Meredith, who moved to the Far North Coast from Sydney six months ago, has bought another second-hand car that has given her no trouble at all.

"I did a lot more research on this car and there were good records on the car – but it's always a bit of a gamble," she says.

Meredith is not alone. Second-hand car buyers are being ripped off as sellers and dealers are concealing pre-existing problems.

An estimated $39 billion worth of second-hand cars are sold each year, but many of those are lemons.

A survey of more than a 1000 people, commissioned by online car finance company ApprovalBuddy, found one in three buyers had bought a car with pre-existing problems that buyers felt sellers should have warned them about. ApprovalBuddy offers free mechanical inspections of second-hand cars.

Of those who experienced problems, just over half noticed the problems within the first month of purchase, with almost half of those saying they experienced problems in the first week.

Fewer than one in five second-hand car buyers told the survey they felt confident enough to spot mechanical issues when inspecting a second-hand car.

Almost half said they had absolutely no idea what they were looking for when inspecting a car, and 7 per cent admitted to "faking it" at an inspection simply to avoid embarrassment.

And repairs don't come cheap. Just over half of respondents with problems spent more than $1000 on second-hand car repairs to fix issues they felt sellers had been hiding.

One in five spent more than $2000 on repairs.

Anthony Simon, founder of ApprovalBuddy, says very few people are able to check the mechanics themselves, which is why so many are buying lemons.

"If you don't know how to properly inspect a second-hand car, make sure you have it checked by a qualified mechanic before you purchase," he says.

Simon says some of the classic signs of problems include oil leaks, which can be because of degraded engine gaskets. As well as being a fire hazard they can cause the engine to fail without warning.

Panels with differing colour tones show the car has been in an accident and has been poorly resprayed, he says.

Another giveaway that the condition of the car is not all that it seems is engine noise.

A knocking noise can indicate a worn camshaft or worn bearing or crankpin, he says.

And transmission noise such as a whining could indicate a failing transmission pump.

"And there's the false kilometre reading. If a car shows a lot of wear and tear but has a low reading, it could be that the odometer has been tampered with," he says.