I don't really know ht eanswer to this but I would be very wary about buying secondhand radar. For one thing the technology has moved on very fast in the last few years and moderate to good quality sets are not outrageously expensive.
I suspect its the mechanicals which wear out first along with the magnetron which are expensive or impossible to replace so any set you buy may have a short and expensive life.
If you get better advice than this and go ahead with a secondhand set, I would suggest buying as good quality a set as possible.
I toyed with the idea of a second hand radar for a while and as I'm sure you're aware used RL series displays come up often on Ebay and sell cheaply, radomes much less so. Often the claimed reason for sale is theat the owner is upgrading to a colour display, credible I suppose. Even if these only sell for a hundred quid or so I understand a Raymarine Radome is 700 punds or so new which puts you over the price of the entry level JRC/Furuno and there's no guarantee it'll all work and no comeback if it doesn't. I also suspect a lot of the second hand marine electronics on ebay are stolen, how many other reasons would there be to remove a fully functioning radar from a boat ? I guess it may sell for more second hand than it adds to the value of the boat but removing the scanner/cable/display is not trivial and leaves some pretty ugly gaps.
Occasionally I've seen an unfitted still boxed item on Ebay when the owner claims to have sold the boat before getting round to fitting (understandable, mine lived under the bed in the guest bedroom for a year before I fitted it !!), one of those might be worth a punt.
As James says, unlike other Marine Electronics Radars contain significant numbers of moving parts, having poked around inside my Radome I'm surprised how fragile the components look, I suspect a bit of water ingress or rough handling and it'd be done for.
I eneded up buying a new Furuno 1623 at around 900 quid at the SBS.
Greenham Marine, Lymington, and perhaps other marine electronics outlets, often put together packages using Raymarine SL72 display units (available due to upgrade) and new 2kW radomes and cables. The display units are bullet proof so you are virtually getting a new radar with a fair reduction in price. This is probabaly the safest way to go if you can't afford all new.