Theoretically does anyone still use, need or want 3DS development model systems with the twl menu and nand filer apps?
Dev units are necessary if one wants to develop and release a 3ds game.
Couple of bad news items though:
1. The retail scene on 3ds is basically completely dried up. Indie devs report total sales of < 100 units with regularity (that ain't going to pay your rent unless you live w/ parents).
2. If you buy second hand, Nintendo won't have record of your purchase and you will still have to buy a dev unit from them if you want to publish (the good news is that their dev program is open to anyone - on 3ds/WiiU at least).
3. Nintendo will also require you to buy a debugger, which is very costly, UNLESS you have a new3ds/new2ds panda, where they will allow you to dev and publish Unity games (not SDK) with just the test unit. It's clear that your panda is an old3ds so you'd need a debugger to publish SDK/Unity games with it. (unity games can run on old3ds, just poorly).
Honestly, the only reason to dev and publish on 3ds is to "prove" yourself worthy of a Switch SDK, a platform where you can actually make a living. (then again, why not just publish a Unity game on Steam first? you'll make more in the end).
The good news is that dev units can play retail games when hacked, so they still have practical value to the general public, and not just as a collector's curiosity. I would put it on ebay for $150-200 and see what you can get.
tl;dr - near worthless as a dev unit but has decent value as a hacked 3ds game player.