I bought a Narco transponder through eBay that had obviously been assembled from cast off parts but was advertised as "working when removed". The seller told me it was my fault it didn't work as I must have damaged it. I filed a claim with eBay/Paypal, returned the unit, got my money back, and entered negative feedback for the seller.
Sometimes, you'll find a unit for sale out of Canada or Europe. Those can be really good deals as you'll have the buyer protection in case it's a dud but nobody else will bid for fear of being ripped off.
To replace that Narco, I bought a Garmin 320 off of eBay and found a used adapter for $50 from an avionics shop up in Colorado using Google. Works great.