Steam has a list of licences I own for the games I have on Steam. They could easily add 2nd hand sales to the Steam store and just transfer that license to another user. No need for a Blockchain when Steam have a perfectly working database.
Why would anyone ever pay full price for a "new" game if there were "second hand" copies that were completely identical to a new version available for less?
I don't care about EA or Activision losing out on that 10th private jet, but second hand sales would absolutely hurt small creators doing great work.
Another way to improve fairness to stores could be to allow them to charge a "transfer fee" of perhaps 5-10% of original MSRP for transferring any used content.