I think it depends on the bag. Much earlier in my career I started travelling for a role that required me to wear multiple suits during 2-3 day trips and I purchased a used Tumi alpha garment bag. I paid pennies for it and still have it (though I don't use it any more). Getting top quality stuff, as long as it is only gently used, can save you a ton.

I would be wary though with rolling luggage or anything with a frame that could have been damaged. The big question is why would someone be selling it. the slightly more cumbersome bags are probably a better bet to pick up used.