I just purchased a 2017 Mazda3 Touring with 40,000 miles, through an online retailer. I am still in my 7-day return grace period. The CARFAX report says it was in a rental program for just over a year, in the Boston area.

I had my mechanic (a Mazda-only shop that I have been using for 20 years) check out the car. He says it is mechanically and electronically in excellent condition. But there is "substantial" rust that is showing on the exhaust, brakes, suspension and under the hood, presumably due to road salt over only one winter. He was particularly worried about the future cost of the electric parking brake, should it seize from corrosion in the future (no indication that it was compromised, just a what-if). He suggested that the value of the vehicle is substantially reduced, and should be reflected in the price. Where his assessment went off the rails for me, is he said the value should be 1/3 of what it would otherwise be (not reduced by 1/3... so $5500??). He also said that it is more rust than he has seen on many 20-year old cars here in the South (Georgia).

I crawled around under there last night. Yes the exhaust pipe is pretty rusted the entire length of the car. But the brake/suspension components and engine/transmission corrosion would never have jumped out at me as being significant. Some rust around the mounting bolts. The brake rotors do look pretty rough, but that gets replaced eventually anyway. But I am no expert, and am used to looking at the parts of much older cars.

Is this a known problem - either with Mazdas, this model, or with cars that have wintered in the NE in general? Is this something that can be remedied or inhibited? Or an issue that will definitely plague me in the future?
I have a few days to make up my mind, but doubt I will be able to find a comparably equipped car in the short term.

 

Looks pretty common for cars that are driven on salted roads. I assume you didn't get a pre purchase inspection? If your not happy, see if you can utilise the 7 day grace period.