Used makeup on the web is HUGE especially for designer stuff.
Definitely in Australia where we don't have great return policies on makeup like the US does.
Often people buy, swatch, work out its not their colour, or the formula doesn't suit them and then re-sell.
Or for limited edition collections or discontinued that were in high demand.
Some MAC eyeshadows I have are listed online MORE than what they were new and they've been opened!
There's disinfectant spray you can use to disinfect make up.
Having said ALL that, no way am I buying someone's used makeup! Disinfected or not!