I've bought a few bags preloved online and in-store and I've never had a problem. In fact, for certain brands I prefer it. That being said, whether you're buying it online or you're buying it from a brick and mortar consignment/secondhand store I would check and see who does their authenticating and what their return policy is. If either one is willing to do refunds/returns don't be afraid to get a second opinion if you'er still concerned about the authenticity. I'd steer clear of anywhere that says once you've bought the bag it's yours no ifs, ands, or buts.
If you're still unsure about buying preloved you should also ask yourself if you think the new reissue is worth what Chanel is asking for. Is there something else you could do/get with the extra cash you'd save going preloved? Remember, the buyer sets the price.