It depends on what it is.

Anything fabric I buy new unless I know it is washing machine safe.
I personally buy tons of wood toys and higher quality brands, so buying used is neccesary or I will blow my budget.
Occasionally my daycare kids bring things from home that they are done with, or even new items that were gifts. I also have lots of family and friends and church members who know how I fly through art supplies, so I get lots of odds and ends from them.
I think really hard about what I buy new. For instance, I just bought some schleich animal figurines. I wanted schleich because of the quality, but I actually could not find any second hand in my area, so I bought them new. I also got a kids' cleaning set new because of the nature of the item.