At what point do you...

I always buy used unless no used available, even then I would wait a while, having hifisearch show me that item or category 'first shown, new first'. check every am, bang: strike like a viper.

I am confident in my skills to evaluate the seller and item and money movement, and so far have not been disappointed. a few things have gone wrong (primarily bad packaging), all resolved, or refunded.

Worried about seller's packing skills, or local pickup only: I use UPS Pack and Ship, seller simply drops unpacked item nearby in your name, walks away. You make your own arrangements with UPS, you pay UPS directly, No need to buy insurance: UPS is responsible for any damage which is unlikely.

If you really want something, cannot find an open box (which get maker's full warranty), or 'safe' used, gotta pay the price. Then I think: "will I regret it? will I regret not getting it?"

For me, anything with moving parts is going to be new. Anything so expensive that I can’t chuck it and caulk it up as a learning experience I will get from a dealer. My system now in comparison with my income is very good… so I am aiming for the very best possible sound… so all of it is new puchased from a dealer (and delivered, first for demo, then installed). 

I almost always buy used. By purchasing used let’s me look at higher end gear. More bang for the buck. Yes,there is usually no warranty,which is the downside. Everyone has a different perspective.  

I buy used/open box/refurbished when it's about 50% or more off retail. Case in point: I bought a Yamaha CDS2100 for $1499 refurbished with a 3 year warranty from a reputable online shop.