Is it safe to install this hot water tank from ebay?
As RedGrittyBrick mentioned it may not be safe. It is not legal in Australia as it is only certified by Chinese standards and the installation of said unit is against the law ( not really a problem for you unless someone else uses it and gets electrocuted, you sell it, or sell the dwelling you are connecting it to. For reference perhaps look up "solar and bamboo faulty tanks")

One of the reasons I would be concerned about the safety is my experience with imported products "waterproofing" certification. 

You will note the plastic union on the outlet of the unit which is there for a voltage break (Scary). 

Also the unit specifies 4mm2 cable, I do not put on that I'm a sparky but this webpage explains How to calculate the draw on cable and considering the example is for 5Kw at ambient temperature and this unit is drawing 6Kw at high temperature in an enclosed compartment I would question the sizing ( unfortunately I do not have the cable sizing code on hand at the moment for AU).

The quality of the controller would also concern me having installed direct import controllers before and see them fail and allow constant heating of the element but you should be able to hop out of the way pretty quick if that were to happen.

In summery I would say not particularly safe but if its only you using it you probably will get away with it.

Un- Watermarked hot water unit installation