You can buy a car seat that meets the U.S. federal standards for about $60. Sure it won’t be as padded or have the maximum safety or have lots of cupholders, but it will do the job and be much safer than what I had as a kid.
You can't get one from eBay or a charity shop because ridiculously they're not allowed to sell them in case they're unsafe (eg they've been in an accident), but I imagine the actual risk to the child of gambling on a second hand one is miniscule.
We've had all ours second hand, 5 in total including ones bought for the grandparents cars, and we've spent less than £80 in total
The expiration dates are to force you to keep buying them instead of re-using what you already have.
Maybe. I do think that the foam they use for impact absorption[1] has a limited life, and degrades over time.
[1] I assume it's the same or similar foam they use in motorcycle helmets, which degrade over time. I personally won't use an old helmet (say, 4 years), even if it has an expiration date in the future (which I also never check - I know when I buy it, so I know when I am going to replace it).
Sarcasm ;-)