Oyster Shells are mainly used in FW setups to buffer kH and maintain/raise pH due to to its calcium carbonate nature.
However, in marine setups, the buffering capability of the oyster shells is negligible as compared to saltwater.
Saltwater has a high pH which automatically buffers your kH to desirable levels (~7 dkH) and together with the coral sand and live rock in your setup, it should be sufficient.
Also, HM is right. Due to its structure and porosity, it is likely that the shells tend to trap dirt and detritus. And reef aquarist often strive to reduce such occurences in order to keep nutrient levels low.