Not sure how to offer a reply, however there are services that suggest prices
based on various criteria and those could be anywhere across the chart...
Model Identifier MacBookPro11,4 (2.2 GHz) MacBookPro11,5 (2.5 and 2.8 GHz)
Model Number A1398
EMC 2881
This one suggests price trends; and I happened across it via a link in MacTracker.ca
database app (a download spec database) their link was in a price, looked like this:

And the current price in blue (in mactracker) opened browser, to this page:
http://us.bidvoy.net/MacBook_Pro_Retina_15-inch_Mid_2015/111422/6m
Sites online that resell products may not have the same configuration as your MB/Pro.
So that would be a matter of adding the other upgrade (SSD?) to the base price then
figuring some discount due to the product has seen use.
Also, if your MacBook Pro (retina, 15-inch, Mid 2015) has an extended AppleCare plan
that should be transferable to the new owner; ask Apple directly about that aspect. And
to prepare the Mac for a new owner usually involves restoring it to its original software.
• What to do before selling or giving away your Mac - Apple Support
Another of the basic methods of preparing a Mac for resale, is outlined here:
http://www.thesafemac.com/how-to-prepare-your-mac-for-sale/
So the pricing may be subjective; some markets may have a higher demand for the item
than others. Companies that resell repaired computers offer used ones with guaranty for
less than a pristine model could bring; so their price structure is not in the same market.
There may be college or university boards online where people list items for sale, be
wary of the usual issues or potential problems with putting your information out there.
Perhaps some Apple User group in your area may have ideas or helpful knowledge.
If there are any AASP (Apple Authorized Service Providers, or authorized resellers who
offer repaired or tested-good previously owned Apple computers; independent business)
you may discretely inquire of the consignment possibilities; that may be limited, or costly.
Each area is different; some have no options except to see if you know someone to sell
the unit to, or ship it away somewhere at much less the actual value, at a loss.
Sorry to have little actual information, and not specific to any location.
Good luck!