How many people here generally buy used?

For me it's a case of the greater the number of moving parts the greater the need to buy new when it comes to audio. 

With speakers there's always the concern with surround rot and invisible damage to drivers (esp tweeters) and voice coils. I think piano at reasonably high volume is a good test for coil damage. You'll definitely know it when you hear it. A tremendously non musical screechy sound.

With amps there might be issues with calibration and imminent dry joints etc. My Creek Destiny amp I bought used soon developed issues with the pre/out bypass switch. Luckily I was able to bypass it with the use of jumper cables.

With the rest there's also the risk of various tweakery applied beforehand too. Frankly I'd rather buy a product as it was designed - not as a failed tweak job.

Having said that there was no other way of buying my current 1970s speakers than buying used. Thankfully the previous owner seemed to pay great respect in keeping it in original spec.

In any case it would always be good ethical practice to declare all mod history when selling. I had to when I sold my LP12.

Despite that the buyer couldn't seem to get it off my hands fast enough.

I prefer to rebuild/restore used gear that might have issues, especially turntables. Currently run a rebuilt Beogram, Nak tape deck and a rebuilt NAD preamp. All other gear is used as well except for a Sony CD deck that I got new way back in 1987.

Rarely buy new audio equipment except a DAC or 3, but don't buy really old either, typically 2-5 years old. I have a tendency to open things up and "play" so like to mitigate my cost exposure.

Usd to buy new in 70s and 80s because reliability was poor. Everything in last 20 years has been used given the ~30-60% savings from new and quality of build. 
I always listen first and try to deal with reputable seller/dealer. It’s was very easy in Chicago but realize this is harder in smaller markets. 
Some great deals I remember:

Pioneer SA9500 amp for 100
Denon 2910 universal CD player for 100 ( still sounds great)
paradigm Studio 20 speakers for 300
audio refinement complete amp and CD for 1000
YBA is integre DT amp for 1500
McIntosh MA5200 for 3100
Sonus Fabre Auditor Ms for 3000
B&W CDM7SE speakers for 1000
REL Strada III sub for 900

just a few I can remember the prices on. 
Even if I had twice or three times the money I would stick with used. 
That way I can buy and sell more often which is the whole point of this game. Right?

oh ya, and the great sounds that make life worth living.