Buying used: Depends on what it is!
Things that shouldn't be bought used: car seats because you don't know the history of them - if a seat is ever in a crash, even just a small fender bender it should be replaced. Shoes. I didn't know about shoes until just recently when I found out shoes change shape to fit our feet - buying them used means they won't form to your child's foot and may not fit right
Other than those two things, I think buying used is great! I buy a lot of clothes, toys, and books used to save on money :)
I don't buy used underwear or bras or bedding usually.