Power consumption is a massive reason to really not do that. Its cheap for a reason, its takes a shitload of power to be shit and you will pay more in energy than you save in hardware unless its only powered on for short periods of time - a server typically isn't.
This is actually something that applies to cheap products too. Was in Asda a little while ago and saw 2 LED bulbs with the same lumen rating. Cheaper one used 3w more and you only saved £1. Running it for 8 hours a day for a year would cost double that saving in electricity. For a server you are looking at almost £2 per watt each year. Does that ewaste look so good to you now?
Some things are absolutely worth getting second hand, but you really should be careful considering the power cost as well.
Quick edit: If you don't need it running 24/7, consider something like AWS too. I love selfhosting but if its not running much it might be cheaper to not bother buying hardware.