I dont think youll get a good deal on a second hand ipad. Its too early in the iPad release cycle for prices to drop significantly new or second hand. People will pay upto the 'new' price on ebay so you might save 20 quid but then you have a slightly used ipad - is it worth it?
But you might get lucky....
You would be better to wait until you have £600 and buy the 32gb 3G and have 'future proofed' yourself to some extent eg, your needs might change and you want more storage on the ipad.