Most merchants that accept bitccoin operate on the darknet, and many of them do not sell legal merchandise. You will need a Tor browser, a VPN connection that hides you IP address, etc.
Are you interested in buying anything specific with your bitcoin, or just planning on holding it for investment puposes? Be mindful that the value can fluctuate wildly, as it did when Silk Road, one of the largest merchants on the Dark Net, who did not sell legal things, was busted by the Feds.