Seems to me that there exists a thriving used market.
I have ever only purchased three audio components new: a DAC, one pair of speakers that were not available used, and a home theater pre/pro. Everything else is used.
With regards to reliability, it differs by type: digital stuff, electrical analog, and mechanical. Assuming that is has been well treated, pretty much anything less than 30 years old is not likely to fail, I think. Good quality digital equipment - things like processors and DACs, are probably going to become outdated long before they fail. Older electrical equipment, which I have owned a lot of, needs a good recap. All things electrical go out of spec eventually. Even then, it's more likely to be amps than preamps or tuners because of heat. Mechanical items, e.g., speakers, transports (cassette, CD, DVD, phono) are more likely to fail sooner, but still get years of service.
If you're concerned about buying used equipment that is less than 5 years old, it's probably not high quality. Nothing well made and well taken of should fail that soon.