There is a lot of sites, gazelle.com, itsworthmore.com, etc. that offer trade in quotes. I would get a quote there and see. I would expect a little more from a private party sale, but if you trade it in at one of these places you probably will have less hassle.
When you use your Mac notebook, its battery goes through charge cycles. A charge cycle happens when you use all of the battery’s power—but that doesn’t necessarily mean a single charge.
For example, you could use half of your notebook's charge in one day, and then recharge it fully. If you did the same thing the next day, it would count as one charge cycle, not two. In this way, it might take several days to complete a cycle.
Batteries have a limited amount of charge cycles before they're considered to be consumed. Once consumed, a replacement battery is recommended. You can use your battery after it reaches its maximum cycle count, but you might notice a reduction in your battery life.
Knowing how many charge cycles your battery has and how many are left can help you determine when a battery replacement is required. For best performance, replace your battery when you reach its maximum cycle count. /