FWIW, you may not have to go the used route for a breast pump if you are in the US. Because of ACA, your insurance may cover anything from an electric pump to hospital rentals to an electric pump. You may have a small co-pay (still less than brand new) or it may be entirely covered. For example, my insurance covered a double electric breast pump 100% (claim amount was ~$250). They’re popular brands too, e.g. Medela and Ameda. If you do have to buy a used pump, go with a closed system (you can Google for more information and models). I would not go with an open system if it is a used item.
I definitely would not do a used car seat. Even a small fender bender can affect the efficacy of the seat. A used stroller may be fine if it’s in excellent condition. If you want to cut down on costs, you can start with a convertible car seat from day 1 rather than transitioning from a infant seat to a convertible car seat. The downside is that you will have to fish out your newborn/infant from the seat every time rather than just carrying the infant seat everywhere. However, if you are babywearing a lot, then this isn’t a problem.
If you are not skeeved out by possibly having someone else’s bodily fluids on your bras, then used nursing bras should be fine (assuming they are actually the right size/fit). Women are technically leaking breast milk onto their nursing bras so it’s up to you how bothered you are about that.
If you are doing glass bottles, then I would consider used as they’re easy to maintain in good condition and super easy to sanitize. However, for plastic, they degrade faster than glass and if there are signs of wear or etching, I would just get new ones.
I personally would not get a used crib because who knows if it was recalled or not. i don’t know how fixed your budget is, but cribs can be found somewhat inexpensively. For example, Ikea sells cribs for less than $100 and depending on the model, they meet safety standards both in the US and in EU.
A lot of things you can do without. For example, a lot of moms forego a changing table. They’ll just change wherever it is convenient – on the bed, on the couch, on the floor, etc.
I would buy toys used if they can be disinfected (in diluted bleach or through the sanitize cycle in your washer). Yard toys (e.g. Little Tykes Cozy Coupe) can be purchased used as well.