Fanwork Ban: Averted. There is a fanfiction section on Sapkowski's official forums (there were also several fanfics featured on the main page too, but are currently unavailable due to reconstruction the site is undergoing).
He allowed Vladimir Vasilyev to use Geralt's name for his The Big Kiev Witcher short story series.
In 2013 The Tales of the Witcher World (Opowieści ze świata wiedźmina), a collection of short stories written by various Russian and Ukrainian authors got to be officially published in Poland with the permission and blessing from Sapkowski.
Keep Circulating the Tapes: Most of Sapkowski's non-fantasy stories he wrote before The Witcher for... angling magazines are believed to be lost, as the author himself doesn't have copies of them. It's up to his dedicated fans to search for them in libraries and put into circulation on the Web.
Write What You Know: As an economist of education, Sapkowski just couldn't resist to include real life ecomomic laws into his fantasy series. Probably the most prominent example is the short story Eternal Fire (Wieczny ogień), in which Geralt meets Dainty, a halfling merchant.