I'm trying to buy a used Japanese New 3DS LL (I hate how Nintendo named that thing) with an IPS top screen, and there's one listing where the price is what I'm willing to pay, the screens are pristine with no yellowing and I'm fine with the condition of the back plates, but there's a small crack in the console just below the upper screen. I do not have the experience to replace the whole shell, is this something I could use a bit of glue or epoxy and be fine or would this become a huge problem?
Epoxy could work.
3DSs have their interconnect ribbons in the right hinge, the left one is 'just' the mechanics.
Also, the shell is pretty sturdy. But nevertheless, you really should replace it, but probably enough just the inner shell of your screen to replace.
Now the sad part is that IIRC for that, you still have to disassemble the whole unit - including removing the ribbons in the hinge then reinserting them. This sucks major ass, it's really-really hard and tricky to do, and there's a relatively great chance you bork some of them (camera, screen or speaker/vol/3d controls).
It's not impossible, tho.
This isn't my 3ds, this is a seller image. There's a lot to like about the rest of it, but ultimately I've decided not to buy this one because of the damage. I looked into what it would take to change it and I don't think I have the skills to do it - and this isn't "fun to tinker with" cheap. And yeah, replacing the shell on a 3ds looks like it sucks major ass, lol.
I've done that kind of replacement before and it's a major pain. I wouldn't buy one like this.
There is no such thing as "minor damage" on a 3DS hinge, since the 3DS only has a single real hinge. The other side is only a hollow channel containing the flex PCBs connecting the two halves. So if the single real hinge starts cracking, the crack will quickly spread until it breaks. At that time the screen will fall off completely and chances are very high that this will damage the flex PCBs. Since they are connected directly to the screen, you now need a new screen.
Using epoxy to glue the crack won't help much (the area is still a mechanical weak point that will quickly start cracking again), and in the worst case the epoxy will glue together both halves of the top shell making it hard to impossible to actually swap the shell for a new one.