I understand about the COA concerns but there are a few of the sellers on there that are very trustworthy. The goodwill of orange county california is particularly good. As well San Antonio, central Texas and southern Nevada.
I've bought over 100 bags on there, with and without COA's. 3 were fake and I was able to return them. None of the fake ones had COA's though.
It really was my fault though, they were brands I didn't know much about, an Etro, Miu Miu, and a Tory Burch and I should have done my research first. The returns though were painless and fast.
A lot of the bags with COAs are from the two top authenticators and have the QR code on them so you can scan them and check them on the authenticators website.