That bass could arguably be "very good," I think it is a stretch to call it anything better. The OP got burnt.
I don't think anyone is debating the actual condition of the bass versus some of the description language. That should have been obvious from the start, before the OP put his money down.
I think the point being made is, and it pains me to say this, the OP let himself get burnt.
Now, onto the next step:
Regardless of the details of the item description, there are sellers in this world who don't want their instrument going to someone begrudgingly who has remorse, or is upset, unhappy, or angry over the transaction. Some sellers would rather just suck it up and accept a return with a full refund, re-list the item (with a bit more honesty in this case one hopes), and try to find a buyer who will be happy with the purchase. A sincere, diplomatic, respectful appeal, perhaps one that accepts partial blame for not interpreting the warning signs, is worth a shot.
Or, if OP likes and wants to keep the bass, you can make a similar appeal for a partial refund. OP and seller can come to an agreement on what percentage discount is appropriate to conclude the sale to everyone's satisfaction.
Short of either of those succeeding, there is also the possibility of OP's credit card fraud/security department getting involved. Then you are at the mercy of the judging powers that be on both sides of the issue (seller will have his version and protectors to contend with).
It's always unfortunate in a transaction when things are different than you thought they were in your mind; now the only thing to do is work it out and hopefully accept the consequences and move on.