Sorry but the software is copyright and you buy a right to use only. So you can't legally buy a second hand card set and use it as you don't have the right to use that software. Also it may not have the feature set that you want (although software this old is probably not bundled). That said, there are many second hand dealers, your biggest problem would be finding someone who hasn't scrapped all the stock of this product as it is pushing 30 - 35 years old. Any spares will be hard to find, especially Dterm handsets.

If you are interested in some advice on how to refurbish the old Dterms, I can help because until recently some major hospitals where I live were still running equipment this old and I had to keep them running. Fortunately no more!