Even if it's not possible through official means, there are other ways. There are ways to install custom firmware on every 3DS in existence, on any firmware version. This is very much unsupported by Nintendo (they condemn the practice) but it will enable you to unlink your NNID while keeping the game on the system.
I'm not a lawyer, but despite what Nintendo's EULA says, I'm pretty sure selling the console like this, at least in the United States, is legal due to first-sale doctrine. Just don't retain a copy of the game or redownload it from the eShop, unless you're okay with piracy and are willing to take the (probably extremely small) risk of being sued. Also, if you decide to go this route, you should explain in the product description what you did, just so the buyer is aware they won't be able to link the game to their own NNID.
See https://3ds.hacks.guide for instructions on installing custom firmware.