Its worth as much as someone is willing to pay for it.
Lets take into consideration the type of buyer for a used machine like this, they are unlikely to be the sort to spend a couple of grand on a new system are they.
So they are more likely to build thier own system, you can build a tank, and ro system from bits and bobs on ebay for around £500 ( if you buy new stuff) and probably £300 for used parts.
So if this was up for sale I'd advertise it for around £350 to £400. Other than that its not worth buying as any more money is the same as building yourself.
You might get £475 for it if you find the right buyer. ( someone who can't be arsed to build his own, but tbh he still has to fit it which isn't much different anyway)