I’m always thinking of buying used headphones. Also i use my headphones daily and for hours at a time. My main concern are the pads. Especially if they are made of cloth, velour or microfiber. Ive done some research on the web and this is what ive found.
A daily or weekly clean recommended is a lint brush (the sticky kind) and a cloth dampened with cleaning alcohol. Wipe down gently. This may be especially good and all thats needed for leather type pads.
For a more thorough clean the best to me seems to get a bowl with cold water use a hand wash mild detergent. One i found recommended is this one:
Knead the headphone gently like dough for about 2 minutes in the water. then squeeze out the excess liquid. pad down with paper towels and let air dry overnight.