You can buy both new and used monitors via ebay.
I have done OK with both.
On a new monitor, look carefully for "perfect pixel" units which will have no dead pixels.
Ebay does protect buyers if your unit is not as advertised.
But, if the unit comes from china for example, returning a unit can be hard.
On used, look for reconditioned units. I would not worry too much about cosmetics, but look/ask about dead pixels.
As always on ebay, know your seller.