Yamaha tend to use CLP1XX and change the first number to show the beginning of a new range. When they get to CLP9XX, they go back to CLP1XX and go through the number system again.
I would say it's a post 2000 model. The current CLP220 is a descendant.
However, the CLP220 has been out for a couple of years now and is about to be updated. I can find suppliers who are selling CLP220s for 650GBP and the CLP220 is a far superior digital piano!! By the time the CLP320 comes out, that value will decrease even more - how much cheaper then, should the CLP120 be?
Be wary about waiting for the CLP320 for the reasons stated already. New models tend to be over-priced and then settle before a complete windfall towards the end of it's novelty.
Perhaps your seller needs to think about his price again!
Aside from this, you can pick up YDP131 or 151 cheaply at the moment as the YDP range is also due for an immenent update, probably due to be released before the CLP range. They are entry level digitals and the 151 model is practically the same as the CLP220 save for the casing. These keyboard are selling within the 350-500GBP price range, brand new that is.
I would say that the CLP220 will devalue substantially over time as the CLP320 establishes itself, but it is both a waiting game and a gamble to an extent. You can see that the CLP120s are still being sold at 700GBP and they really shouldn't be as they're technically obsolete.
What you could do is continue to monitor online prices for the CLP220 and wait for the launch of the 320 (which should be out really soon) and compare the keyboards. I hear that 5 layer sampling features in the new pianos (or at least some of them) which may be worth considering. They will be dear when they first come out and it will be hard to get a reasonable price (note, not a cheap bargain!) in the CLP3XX range, so take that into consideration before pinning too many hopes on getting a bargain for a new CLP320.
Beside internet, consider secondhand models too. Many fanatics may be willing to part with their CLP220 cheaply for a quick sale in order to get the newest model. Sounds strange, but those folks are out there! Check your local paper too. I'm reluctant to recommend Ebay purchases in digital instruments, as I always advise people to try the instrument before buying and this is even more the case when purchasing secondhand! If you decide to go down that route, be careful that the piano is really what you want and that it is in good condition. There is no substitute for trying the piano out yourself, though!
Woodgrain cabinet with dark rosewood finish
88-key graded hammer effect keyboard
New Stereo Sampling AWM (advancing wave memory) tone generation
30 Sampling Banks
64-note polyphony (Max)
PC Interface (TO HOST)
Digital reverb, chorus, brilliance
2-track song recorder with 1 song capacity
20W x 2amplifier
3 foot pedals
Sliding key cover
Dual headphone jacks and headphone hanger