Don't throw a baby shower or any other social event if the mother-to-be doesn't want one. Maybe she is very uncomfortable in social situations or has some difficulty she doesn't want anyone to know about. You can't always tell by looking either, don't assume that because she is able to cope or cover she couldn't possibly have any issues. I consider any kind of event at which I am the "center of attention" roughly equivalent to the seventh circle of he.., well you get the idea.
You don't need a shower to send a present, if you want to send a present, do it. Encourage the people you would have invited to do the same, if you have occasion to speak to them.
It's very sweet and thoughtful of you to consider the thrift store issues, you are correct, a used car seat may have been recalled (which you could easily check on-line), but worse, could have been in an accident already or somehow dropped or damaged and should be discarded (which you can not check).
If you want to do something fun (for everyone, not just the mom-to-be) have a "let's swap stuff" party. Everyone brings a treat to eat and multiple copies of the receipe and whatever nice things their kids have grown out of or they can't use anymore.
No kids are invited (this is very important, since things for kids are up for grabs). If someone doesn't have used stuff, they can either bring new stuff or enjoy the party snacks.
If people know a mom-to-be is going to be there, she usually ends up with a lot of cool stuff for a newborn.