Under EU jurisdiction, resale of software licenses is legal, as established by the EU court in 2012. See
http://www.computerweekly.com/opinion/E ... d-software for a brief summary. I don't know what your legal position is if you are outside the EU.
Concerning Sonus Paradisi, some of their sample sets are unencrypted, particularly the older ones. These sample sets are vulnerable to illegal copying. Resale of these sample sets is not supported, so you won't get updates as a second-hand buyer. I think the part of the license agreement that prohibits resale is void under EU law, but of course, selling a copy and retaining the original is piracy, illegal and criminal.
Most of the more recent sample sets are encrypted/dongle protected, and can be officially transferred to others via your personal account on the Sonus Paradisi website. A small administrative fee is applied to this procedure.
Most other sample set producers have encrypted sample sets with similar provisions.