You can buy a parrallel, serial or USB dongle if you are sure that the attached licenses really belongs to that dongle.
The first thing to do is a license swap from the old dongle to a new SD card and when the swap has been don you can buy any license you want for the new SD card, no problem.
You cannot swap licenses from a old dongle to a new dongle wich already have license attached to it.
As said, you need to be 100% sure the dongle really has the licenses as the seller says. Befor buying ask for the dongle serial key and call a distributor for a license file for it, that is the only way to be sure you get what you buy.
So basically yes, old dongle are valuable if the licenses are still current like VM Pro , SoftConsole, PhoneManager Pro, VCM channel lics, Voice networking lics etc.
Obsolete licenses are Upgrade to Proffessional, CCC licenses ( more or less, you need to buy a CCR upgrade lic to use the CCC lics ) VPN Phone lics, Compact Business Centre, DECT Itegration and a lot more. you could ask here there are some nice guys able to help you.
Be informed....
If it ain't dutch it ain't much