Firstly, I wouldn't recommend buying a sniffer on ebay. The reason is that the sensor is ruined if it comes in contact with oil of any kind. This would seem to make this a risky proposition when buying one used and unseen.
The same reason keeps me from buying one, that and that fact that UV dye has worked so well for me. The UV dye is no problem for 134, it is readily available in a can about anywhere, just make sure you purchase it with the correct oil type, PAG or Ester.
I have an injector that uses a pure dye and is compatible with 134, 12 and any oil. It cost about $60 at my automotive a/c shop.
I have a UV lamp and yellow safety glasses and it has yet to fail for me in diagnosing leaks. Most leaks are in a place where you can see them and if it is the evaporator, you will see UV dye in the condensation water that drips underneath the car.
Personally the UV dye works for me far better than a sniffer.