Available for purchase here: https://www.unrealengine.com/marketplace/optimized-rocks
Some personal notes: I had taken great pains to create some beautiful and highly optimized rocks for my game, when the prospect of selling them was presented to me, prudence dictated I moved toward that end post haste. So I prepared them by fixing various details needing polish, cleaning up UVs, fixing normals etc., and even upgrading them by including a new distance fading normal material!
Submitting the pack was daunting, as I had heard Epic could take up to a month or more to assess submissions (and indeed I experienced that very time frame), also the thought of rejection was a bit stressful, but after 4 re-submission requests to correct various technical aspects and obscure guidelines (naming conventions and other minor things), it was accepted! For posterity I want to mention the road was not an easy one. Each iteration came at the cost of more time waiting for a word from Epic, but having now become a veteran of the process I understand the reason for Epic's stringent adherence to guidelines and agree with it. A comforting sense of accomplishment is awarded for the efforts!