I wouldn't jump to the conclusion that the bag is fake. I've seen some very old LV bags on eBay that were just beat to Hell...rips in the canvas and everything. Some people just don't care for their stuff well. I would get it authenticated here before taking it to the LV store for repair. That way you can save your trouble if it is fake.
Older LV was very well made, they hold up quite well, and if something happened to be a miss with the bag it's likely one of those issues rather than both at the same time.
I'm not sure how often you've seen counterfeit bags up closely, but I have noticed without fail no matter what style it was a copy of, the zipper and the handles are the first thing to go and often at the same time.
Goodwill has started to auction designer goods, so if they feel it's authentic, even if it's shot to hell, they will auction it online rather than sell it in their store for $10. So if they forgo the online auction and sell it in store, it's probably not a good sign.