Value of the slider MADAS with respect to the key MADAS is a different ballpark, and the quoted prices on ebay are ridiculous in the first place - which is why these machines have been there for months. They could realistically sell at around 900-1200$. For a late electric keyboard MADAS in this condition, you may get 400-600$ on ebay, if you are lucky. Keep in mind that the one linked by Rick that sold for 1450$ has all the keys, is a manual crank model, is in great condition, and is in the UK - nearly all the collectors for these machines are in Europe - machine in the UK = no customs charges, and less expensive shipping - so price can go up by 35%. Look at
madas.ch for a flavour of what else is around from this brand - there are plenty of different combinations of capacities, many of which are much more rare than others, which also drives up the price. This is a IXeT MINA. Not one of the commonest models, which is good, if you can sell to someone who knows what they are doing. In any case, they were (and are) very high quality machines.
If you got it cheap enough, you'll do fine on ebay. If you paid a 1000$, basically don't expect to get your money back.
I would recommend to our US friends on this list to try owning one of these machines - the quality is fantastic, and once you have gone through the electrics in order not to get killed by AC due to lacking insulation and grounding, they are a joy to run.
Cris