Flori I'm sorry to report, they do unfortunately lose their value. This is because people buy newer machines for their practises in order to give the best treatment possible. I've seen machines like your Fisher scientific sell on kijiji for anywhere beteen $150 and $400 but usually not more than that. The last kijiji ad I saw for an SE4 stayed on the site for months here in ontario, and was only priced at $140. So the demand, isnt really there.
What you may consider however, is offering the machine for sale here. While not useful in a professional practise much anymore due to it's age, they are very helpful for DIY'ers and beginners and we often get people wanting to attempt DIY here that cant find descent machines in their area.
Ebay is a total crap shoot as to final value from it, but that is another option. I wouldnt expect to get more than what I stated for it though.