Sorry to hear about your misfortune, but..
How about the shop that issued the airworthiness tag on the part? You should be able to contact them and get a copy of all the work orders documenting the work (and bench tests) performed on the unit.
If you find the tag is bogus, contact the FAA. They (and the FBI) would like to have a chat with your seller, I'm sure. You may not get your money back, but you may save someone else from a similar fate.
http://www.faa.gov/aircraft/safety/programs/sups/
HO5S-1