Be careful buying a car seat used. It needs to have never been in an accident. You also need to watch the expiration dates (printed on the car seat, usually they last for about 6 years).
There is a fair bit on used crib mattresses. Read and decide.
The only thing I would suggest new is a stroller if you will get hard wear from it. I wore the rubber off the tires on mine in 20 months (we walk like an hour, every day).
There rest is wonderful used. I know more...