I've bought over 30 used basses from GC. No problems as there's that 30 day return policy if there's anything wrong with it. You can just return it to your local store regardless of which GC store in the country you bought your bass from. You will usually get the shipping charge deducted from the refund if it's because you simply don't like it. Shipping charge is around $20 or less - price depending on your distance from the store you're buying from. I've had it shipped from Portland to Michigan and it cost me $23 shipping, Ohio to Michigan was like $13.
Always call the store and have them give you a description of the condition of the bass over the phone, sometimes they can even photograph and email you additional pictures of the bass if you're concerned about something.
Regarding the ratings.... it's pretty inconsistent as a "good" condition bass may be considered "fair" at another, so you really have to get a verbal description in order to get a better idea. Ask about the overall condition, the neck, electronics etc.
Shipping to your door by the way, is pretty fast. The longest I've had to wait was 4 days, the soonest, 2.
I highly recommend buying a bass from them if you find one you like, it's really not at all risky because of that return policy and you do find quite a selection you may never see at your local GC store.