If it’s stolen they should contact law enforcement in the countries in question.
That's the first point that needs to be clarified. What they've said is "sold without authorization", which is a vague term that could mean anything.
The fact that you've been approached by a garbage disposal firm and not a law enforcement officer may be a clue here, but I'd still press the point. Maybe respond something like this...
Dear <name>
Re: your recent letter concerning my test equipment
I understand that you would like to take possession of my <device>, which I purchased in good faith from <supplier> on <date>.
I feel it is important to clarify the precise legal status of this equipment before proceeding any further.
If the item in question constitutes stolen goods, (ie. is subject to criminal legal proceedings), then I will of course return it to the rightful owner as soon as I have received a signed statement to that effect from a Director of the company which owns the equipment. In this event I will, of course, ensure that the appropriate law enforcement agency is informed of the incident, and I may pursue the seller for any losses and/or damages.
If, however, "without authorization" simply refers to a breach of contract between two parties (neither of which is me), then please understand that I am under no obligation to return the equipment whether for "fair market compensation" or otherwise, and that I have not yet agreed to do so at this time.
<Delete as applicable...>
[The equipment is in regular use and is essential to my business, therefore I cannot contemplate returning it at this time unless compelled to do so by law.]
[The equipment is in regular use, but I would be prepared to relinquish it in exchange for a brand new unit of equivalent specification. My <device> will be made available for collection by you within 7 days of the arrival of its replacement. All taxes, duties, shipping and other costs will be paid for by you.]
[The equipment is not currently up for sale, but I would be prepared to consider an offer of $XXX plus shipping.]
I look forward to receiving your detailed clarification of the status of the equipment in question, and of any offer you may wish to make in exchange for it.
Yours sincerely,
......