A+ Auto Mall

On March 11, 2013, I purchased a used (2010) vehicle from this dealership. After I drove the car home that evening, the check engine light came on. I called the dealership the next day and made an appointment to have it looked at. I took the car in on March 13, 2013. After hours of not hearing from anyone, I decided to call the dealership. I was told on the phone that the check engine light was on because the car was sold to me with an aftermarket exhaust and headers (which I was not told about when I bought the car) and that it would not pass emissions because these parts were causing an O2 sensor error code. I went to the dealership to pick up the car. I was then given a paper detailing the results of the failed emissions test. Everyone at the dealership denied knowing that these parts were on the car and claimed that I "knew they were on the car when I bought it", which is false. I was never told anything about these parts or the emission testing until after I bought it. After looking over the paper more carefully, I noticed it was dated March 7, 2013. Why would the dealership give me a paper dated March 7, 2013 when the test was supposedly done on March 13, 2013? Either they falsified the date on the emissions paper or the emissions test was actually done on March 7, 2013...which would mean they did in fact know that these aftermarket parts were on the car and that it wouldn't pass emissions. After speaking with the sales manager, service manager etc., their solution to the problem was to get the stock parts back from the previous owner and for me to return the aftermarket parts to him. First of all, how do I know where these parts from a car that's three years old were stored or if they are in good shape still. Secondly, why would I just give away parts to a car that I paid for? The dealership wouldn't even have a second key made for the car and I was never given an owner's manual.