I've been held responsible for they unproffesional

I call this shop for several time looking for a part all they inform me about was the price of R5 000 and that the part is available, on the 05 of February 2016. I went to buy the part in that time I informed them that I am not sure about what I am buying and the offered a slip and 1 of them marked the parts for them to know that its from they shop. they didn't tell me about they terms and conditions on they are goods. I never checked the goods because I have know idea of it and I was exited that I got it at last but I thought I was safe because I informed them about my situation and I have a slip with me. then I was told by my mechanic that the other parts are wrong and only 1 parts is needed, then on the 08 of February I called the shop to inform them then they agree that they will take those and give me the other 1 with R3 000. on the 9th of February I went back to the shop for the agreement when i get there they told me that they don't return the electrical parts, that the person who was helping me which is one of they are staff was wrong for making agreement with me and they have issues with that kind of behavior. and they are not sure of the part anymore because its opened .