I prefer to buy new directly from the manufacturer, but I wait till the bag I love goes on sale.
I've purchased bags from ebay but they almost invariably arrive dinged, scratched, scuffed, smokey-smelling and not-as-described. I've also been noticing that some of the bags I've been stalking have been appearing on ebay at prices significantly higher prices than on sale sites like smartbargains.com and even at prices higher than on the manufacturer's web sites.
When I do purchase bags from ebay these days they're mostly well-loved vintage ones.