As long as you can't insert a SIM Card into the 3DS, which is very stupid since there is a hotspot feature in every device.
To enable the Hotspot on your phone go to Settings > Network & Internet > Hotspot & Tethering > Wi-Fi Hotspot > Turn it on and set your own Wi-Fi Hotspot.
Is this for Online Trades or Local Trades? If Online you can do that from using your Wi-Fi Hotspot from your device, if Local there is no need for Wi-Fi Hotspot.