eBay alternator update

I bought a 'thehan13' 100 amp alternator on eBay less than 2 years ago and it has now died. Got really hot, burning smell and voltage drop. Put the original back in for now and no problems. These are a bit dearer now too so watch out.

I brought a 100amp alternator off eBay just over a year ago and in the past couple of days the voltage drops down to 12.3volts and after a while it comes back to 14.2 volts. On the weekend it dropped down to 12.2 volts so I tried 5 different alternators and 2 batteries and still 12.2 volts I rang an auto electrician he told me to take the car to him so I put back my alternator and battery and when I drove into the auto electricians driveway it came back to 14.2 volts the auto elect tested it for 1.30hrs and couldn't find anything wrong with the car and it happened again this morning
I need help