You could build it yourself for not a great deal
600 ltr tank £200
Two 40-40 RO's £500
Two Particulate and Carbon Filters £80
Two DI tanks £100
Two pumps £120
Leisure battery £80
These are guestimates of prices so don't quote me but they'll not be a million miles off.
If you were to get it for the £750 starting price then it's a decent deal, wouldn't expect it to go over £1000
It looks like a car dealer who is selling it, he'll have lifted it out of a van they've bought.