If the map is purchased on an SD card, then it will be locked to the SD. So there's no reason you can't buy it, use it, then resell it. But it's really hard to find Tomtom maps on SDs these days.
Most commonly now, maps are purchased and downloaded via HOME. If purchased via HOME, the map will be locked to the Tomtom device's serial number, meaning you can use any SD on your Tomtom, but can't resell it for other people's Tomtoms.