Fanwork Ban: Averted. There were fanfiction sections on Sapkowski's site and forum, but they got deleted when the whole site undergone a reconstruction (along with many other things, so it wasn't an operation aimed at fanfiction). They can be only accessed via Web Archive. It's probably still safe to create Witcher fanfics (of course, knowing the author's attitude it may change at any moment).
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.
Money, Dear Boy: Sapkowski always was very open about writing predominately to cover his own bills, but by Season of Storms he outright called it "a small money-printing machine".
One-Hit Wonder: No matter how to count it, none of his works gained as much attention, recognition and basic publishing output as the witcher saga. And that was before video game adaptations reignited popularity of his works.
Trolling Creator: His attitude toward fandom and its expectations when still working on the witcher saga is best decribed as simply mean-spirited. He openly declared few times in early 00s how the infamous decision to Torch the Franchise and Run was made solely to get rid of fans, as they annoyed him.
His short story Battle Dust is an excellent example - he wrote it as a fragment of a non-existing Space Opera book and many fans have been fooled into thinking he is actually writing one.
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 Flame (Wieczny ogień), in which Geralt meets Dainty, a halfling merchant.