Hello Chris,
We (Milan Digital Audio) do allow our sample sets to be re-sold privately (but not advertised for sale on this forum), and some other sample set producers have end-user licence agreements that allow private re-sale too, but not all. (I don't think Sonus Paradisi do, for example.) If in doubt, please check with the maker(s) of the relevant sample set(s).
If you do sell a sample set (by a maker who allows re-sale) then please contact us privately and we will arrange with you to update your USB key to remove the sample set's licence from it, and with the buyer to update his/her key to enable it.