my gut feeling is to return it because I think it will forever bother me.
Unwind the deal completely. Return the bass and get a full refund by any reasonable means (eBay, PayPal, your bank, etc.)
Anything less than that will leave you with a nagging bad feeling about this bass for as long as you own it (which probably would not be "forever").
Legitimate questions about ethics, utility, monetary value, and eventual resale aside, what is important is that this is a musical instrument that (I assume) you intended to play regularly. You would be holding this emblem of ill will and bad feelings, trying to make beautiful music on it with your hands. That's not a great way to live.
The higher road is detachment and positivity, wherein you would keep the bass and get over the moment of badness associated with it. But unless you foresee keeping this bass long enough to discover whether you can take that path, now will be your only best chance to undo the deal and just start over.
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