Well it really depends on what kind of bags you are looking for. If you are looking for designer bags outlet stores are amazing. I am not a huge fan of buying designer stuff online especially ebay unless they guarentee authenticity...at least in the stores you actually see if it's real.
Ebay is the largest online shop but i am not a fan as i rather go to the mall and buy it myself rather than order it and wait...search through google and you'll find lots of online stores...