I really wish I wasn't away from my computer all day so I could model this in excel and format this post a lot better. I apologize if this comes off as me rambling. Feel free to skip to the last paragraph if your not interested in the numbers.
The $50 you'd get for returning the battery is close to what you could sell it for on eBay before shipping and fees. After fees, your looking at $35-$40 profit from the battery.
Your quotes for shipping and fees are a bit off. This is going to weigh around 13 lb., so your shipping quote is $5-$10 low. eBay plus PayPal fees are approximately 13%, so assuming you sell for $200, your looking at $26 in fees. fees everything else is good. If you go the eBay route, your looking at $20-$25 in profit (not including sales tax from purchasing the kit because I'm not if its deducted from income tax). All in your looking at $55-$65 profit.
I've had an M12 Kit posted on wallapop,offerup, 5miles, letgo, and craigslist for about a week and have yet to receive an offer above $170. I've received several for $150-$160. If you return the battery, your into it for $85. So, your looking at $65-$85 locally. If you keep the battery, this turns into $15-$35 profit. Assuming you make $40 from selling the battery, your looking $55-$75 in profit.
All options turn out to be fairly close. What makes returning the battery and selling the kit for $150-$160 the better choice is the fact that you should be able sell it within a day or so. This allows you reinvest your money at a much quicker rate than any of the other options, while making approximately the same amount of profit.