I'm pretty sure you have to buy a license for nomad software (instead of the offline version downloaded off the website) to get any output... regardless of interface. Here's a link to a 256 channel ETC gadget - not bad of a price point for ETC, IMO. Seconded, you probably won't find any used. New would be the best option.
You may be able to find a used Nomad puck, which has the software + license already preinstalled- pricier but you may find one used...
I don't think the used market will ever be very strong for these. At their price point I imagine a lot of people will hang on to them for backup even if they aren't using them anymore. The only ones that would be popping up now would be people that bought them and didn't like them, and they will probably want close to full retail.