Car seats expire because the plastic shell deteriorates and becomes brittle. Not because the seat companies want to make money or because they get dirty.
The plastic becomes brittle and can shatter in a crash. I have seen videos where the harness itself gets ripped out along with the child. There are very inexpensive options available in big box stores all over but if you cannot afford those then check out your county/state department of health or safety or the PD.
You should cut the straps on that seat and cut up the padding as well so no one else uses it. I have heard people say that they find seats on the side of the road and sell them or use them. It is best to destroy the seat so that no one else uses it.
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