I take it you purchased the item from Outback Trading Company.
It sounds like the piece of equipment contained some proprietary software and shouldn't have been consigned to Outback for resale. So they're tracing the equipment forward -- e.g. they probably contacted the person who gave it to outback, who told them "i gave it to outback", outback told them they sold it to you, and now you're getting the letter.
I'm not familiar enough with IP law to say whether or not they have a leg to stand on, but my guess is the original purchaser probably had a contractual agreement that the device would be returned or destroyed and not resold.... in which case, an argument could be made that the OP didn't have clear title to the equipment to 'resell' or consign it to Outback for sale. This may have been a complete oversight on part of the OP -- e.g. if an employee acquired the equipment with the restriction, left the company, and the company then unknowing of the restriction liquidated the item.
I would offer to give it back to them in exchange for a brand-new spectrum analyzer meeting the specifications of the one you have. It's a win-win for them -- they get their IP back and you get a brand new SA. Far cheaper for them too, in terms of litigation.