Is it ok to buy a used car seat?

No. I would advise against it. I had at least one family find bedbugs in a used crib, and car seats are not much different.
No. I'm all for parents trying to save money, but in this case unless you really know the seller, it's best to buy a new one. Any car seat that's been in a significant accident can be compromised, and may not offer good protection-- they're supposed to be discarded. But you never know if you're buying at a garage sale or thrift store what that car seat has been through.
Yes! It's a great way to save money. There is minimal risk of bedbugs because the framework is mostly plastic and metal. Like any used objects, inspect carefully for defects and in the case of car seats...Check with manufacturer that it was not recalled for any safety issues. The fabric piece is easily removed for washing or replacement.
Check around. Many local charities/programs/fire or police departments/ civic groups/ health plans/ etc offer car seats for free.
No. Used seats can have hidden problems. Seats are considered bad if: They were ever in a crash, occupied or not. They have substituted parts, straps, buckles etc or visible repairs. They are past their usable age. The stickers on the sides are gone. If you get it from a friend who has all the parts and none of the above applies, it may be ok. You could have it inspected by a CPST Tech to be sure.
No. Car seats are like beds. We do not tend to buy used mattress. Aso be mindful of numerous recalls and most importantly the new american academy of pediatrics guidelines for infant and child car restraints. The new guidelines for example, call for kids to be rear facing until age 2. No doubt changes will be made in the car seats themselves.
Yes. If a carseat is fairly new, and is not damaged, it should work just fine. A parent should be able to thoroughly inspect the used carseat to be sure nothing is broken, cracked, or missing. If a parent has very little experience with car seats, it may be wiser for him/her to just buy a brand new one.
No. Need to check year that was made and seat belts there are some concerns that when the car seats are older than 3 to5 years old may not work well.