Hello Scott,
Whether a sample set is allowed to be re-sold depends on the sample set producer's licence agreement. Milan Digital Audio samples are allowed to be re-sold, for example (although listings for second-hand sample sets aren't allowed in the 'for sale / wanted' section of our forum).
If the sample set is licensed via the USB key then transferring it to another user requires several key updates via us (one to remove it from the seller's key, one to verify that the removal update has been applied, one to add it to the buyer's key). If somebody does agree to sell a sample set privately then the seller needs to contact us privately to let us know the details, including the sample set name and the buyer's name and email address, and we will then arrange the licence transfer accordingly.