My Suggestions Are:
Yamaha TG33 - Super cheap vector synth by Dave Smith - Brilliant Strings - as used by the Pet Shop Boys
Yamaha V50 - one of best examples of Yamaha 4OP FM around but forgotten and currently probably worthless
Roland D-110 - A real synth in a cute little rack with terrible UI and factory patches but capable of some very unique sounds
Please note the above are only worth it as cheapies - I love them but let's not try and talk them up into anything valuable on eBay!

And definitely where the parts are worth more than the synth....
DO NOT throw away Kawai K1's - their keys are the same as the almighty EII and hence worth real money!!