Occasionally, depends on the item, condition, price and risk
I frequently buy refurbed network gear, like routers and usb adapters. So far, the routers (mostly Linksys) have been rock solid. Some of the adapters have been a little quirky. I purchased a used processor and power supply for an elderly Sony VAIO (about 2001 vintage) and they are still running. On the other hand, I recently bought a refurbed usb harddrive. It came up as a USB mass storage device, but the file system wouldn't open, so I returned it.
I refurbish/repurpose donated used computers for my church. I try to do it as close to $0 cost as possible. A lot of the donations were on their last legs, though, and the harddrives failed within about 3 months of putting them in use, so I don't do too much of that anymore. I don't usually buy used computers for the same reason.