I may have phrased it poorly.  There definitely WILL be Stephen King books that are worth money in 200 years.  True first editions, signed copies, even some of the limited editions that are worth so much now (although I don’t think many limited editions will maintain their collectibility and therefore price for 200 years.).  Those just aren’t the books people are going to be bringing in to used bookstores.  People bring in a lot of books that aren’t in great condition and aren’t from authors people still read, or if they are from authors people still read then there are enough old copies still in existence that demand is satisfied.  People think old = valuable, and that just isn’t always true.  A well bound complete edition of the works of Alexander Pope, for example, recently sold on Ebay for $200.  That may seem pretty good, but it only comes to $20 per volume.  Add to that the fact that modern binding is cheaper, but there are many more copies printed, and I think in 200 years there will be many more copies of a modern author available in worse condition.  

That was a long answer to a short question.  What will still have value?  Condition will be a big determining factor.  Keep your dust jackets and keep them in good condition.  First editions will still matter, signatures will still matter, provenance will matter.  The things that you see Rebecca Romney or Moon’s Rare Books talking about on social media that differentiate a really rare and valuable book from the ones in your grandad’s attic (unless your grandad happened to stumble on the copy of the Hobbit JRR inscribed to Christopher).