With 0.5% full scale relative accuracy, it is one of the better meter out there, definitely better than 1 or two % you see in regular hobby meters.
The salinity scale is between 0.0 to 80.0 ppt, which makes it practical for monitoring salinity in aquarium water (~35ppt). The conducitvity is has resolution of 0.1 uS in the smaller range, so it is fine for RO water too. Like most thermometer, the accuracy is +/- 1C (I hate it when people give me reading like 79.8F on a $10 digital meter, it is only good to +/- 4F anyway).
What really makes this meter expensive is the RS232 interface, but unless you can take advantage of that, I will said this...as long as the bid does not go overboard, it is a pretty good deal below $150. If someone uses the RS232, expect him/her to bid $200+ for it.