Mazda3

My wife has a 2007 Mazda3 s sedan.
We bought it used with 25K miles on it.
When we looked at it at the dealership, it looked pretty nice and clean.
Over time, things started going bad with it.
First, the right front strut leaked and lost all of its oil.
Then our mechanic found that the transmission fluid was dirty and needed changing.
Now, the check engine light is on, sometimes.  We checked, and it turns out that the oxygen sensor is malfunctioning   We need to get a new one, but it costs something like $250-$300 at the dealership.
We found a compatible sensor on Amazon, Denso 234-9085 Oxygen Sensor (Air and Fuel Ratio Sensor), but when we got it, it looked used.  We took it to our mechanic, and he said that the brand was filed off and that it was fouled with oil.  We returned it, and were promised that Amazon will check their warehouse to make sure that the sensor that they are selling as new is in fact new.
The same thing happened two more times.  Last time, the sensor came and it looked clean, but the brand was filed off, so it was probably re-manufactured.  We returned the latest one as well.
We will try again, probably, because on Amazon this sensor costs less than half of what the dealership charges.