It will absolutely NOT appreciate in value, but being a factory sealed MBP means that it has a high resale value right now. However, realize that Apple is releasing new MBPs every year. So since that 2015 MBP was released, there's been the 2016 with Touch Bar and now the updates announced in June. So your 2015 is now 2 "generations" behind..
Most new buyers will likely want the Touch Bar and/or the Kaby Lake processors on the 2017 update..
So look at eBay or other similar sites to see what the price of a MBP like yours is going for, and you can add a little bit extra because yours is factory sealed.
These Macs are not collectables and as such will not increase in value, if anything as technology continues to improve quickly, the older Macs will be outdated and useless..