@hysterical We bought some things new, but only what we couldn't source used (except for safety items- the items others have noted, but also nipples for bottles and cleaning tools should be replaced every so often due to bacterial buildup). Otherwise, why would I buy the 12 onesies, per size, I needed to handle the times they decided that middle of every feeding was the right time to spit up on their chest (and mine) when they size out in some randomly decided window of 2 weeks and 4 months?
The majority of the things we bought new (or were gifted by others) were largely things that we were very picky on and just couldn't find used (or, in the case of stroller #1, we found end of year model clearance sales that dropped the price to $20 more than new for that stroller, so why not?).
Across 2 kids for the first 2 years of each of their lives, this amounted to a number of car seats (actually got both infant seats for free from folks we knew and trusted, but we needed more bases and the older kid seats for 2 cars), a single crib mattress, some cloth diapers/covers (even if you use disposables, get the covers- so much easier to clean that after a blowout than the rest of the outfit), a stroller and some accessories for it (got a killer deal, then bought the second one used on almost as good of a deal), our video and audio monitors of choice (timing meant they weren't available used yet, since everybody who had bought them early and loved them were still using them), a ton of bottle nipples (kid #1 was in the NICU and strongly preferred the bottle, kid #2 was not in the NICU and strongly preferred straight from the taps, so we actually gifted onward a number of still in packaging nipples from the second kid), and a truly random smattering of items (sound machine, night light, a few one off items of clothes we couldn't find locally, etc.).
We still get used a lot, though as the oldest is growing into size 6 clothes some things are getting a bit more scarce (shoes and seasonally appropriate boots are the hardest, even with planning ahead and buying when we see them available) so we're having to buy new here and there.