Blackstone Group LP and Thomson Reuters Corp. are considering an initial public offering or a sale of their stake in the bond-trading platform Tradeweb Markets LLC, whose users and co-investors include the world’s biggest banks, people with knowledge of the matter said. The two firms have held talks with the bank shareholders, which include Deutsche Bank AG, UBS Group AG and Royal Bank of Scotland Group Plc, about selling the company or taking it public this year, said the people, who asked not to be identified because they weren’t authorized to speak publicly.