Mil problem with car seat

I usually get along with mil and we hangout and talk. She was telling me that she was going to a used store to buy a car seat for the baby. She said they sold new seats there. I told her I wasn't comfortable with her buying a used seat because they expire and could be unsafe. She argued that it would be new but they sell it at the used store. Idk what to do. I am now reluctant to let her care for my baby because I will be feeling insecure about the car seat she will have In her car. She doesn't understand me. Someone could just have taken care of the seat well, cleaned it and kept the box and claimed its new. I feel like I'm going to have to be harsh with her and tell her that if she doesn't buy the car seat from a store that I know sells only new seats then she can't babysit or take care of my baby. My husband agrees with me. How do I tell her nicely! Or any other recommendations? I just want peace of mind.