Commodities broker Marex slips on secondary offering launch

** U.S.-listed shares of Marex Group MRX.O fall 3.8% to $37 in post-market trading on share offering launch

** UK-based commodities broker says certain selling shareholders to offload 8.5 mln shares

** Private equity firm JRJ Group, which owns ~14.4 mln shares , or ~20% stake, looks to sell ~3.6 mln of shares being offered, per the SEC filing

** G oldman Sachs, Barclays and Jefferies are lead bookrunners

** MRX shares on Mon closed up 2.8% at $38.47

** Marex went public last Apr after co and existing investors sold nearly 15.4 mln shares in IPO priced at $19

** Earlier on Mon, co told Reuters it bought UK cobalt trader Darton Commodities, while declining to provide more details