My copy of Fire Emblem is pre-installed already on my 3DS XL so I can't sell even if I tried. I think this game still has a lot to offer. I want to try and get ALL the support conversation unlocked and there's also the inclusion of some DLC maps. I might hit a wall and realise I've played everything the game has to offer but even if I did have the physical copy, I wouldn't sell it.
Like the previous poster, I as well am too busy replaying it. Beat it first at "Normal-Casual", now playing it in "Hard-Casual", after that I'll probably take a look at "Extreme-Casual" (I don't see the need to make the game even harder with playing in "Classic" mode. Not saying others shouldn't, if they have fun with it, go ahead).
What I love is, that you can recruit your "old" character from a beaten savegame in your new savegame (if you have the money that is, mine for example cost 80600 Gold). While typing this, I'm wondering if both old characters could be recruited in a third run. I guess I'll see when I beat "Hard-Casual" sometime, currently I'm more playing MH3U (which is just awesome btw).