The selling of "pearls" from giant clams seems to be a fad these past few years. It is common for giant clam shells to be worked (shaped and polished) in order to pass as pearls. But they are not actually pearls. Even though shell and pearls are made of the same material, shell is not pearl.
Sometimes an actual pearl is found but it is often removed from the shell, destroying any value it may have had to collectors.
In any case there is very little market for such items as they are not gems, not beautiful. Also, giant clams are a protected species. And I do realize some of these clam shells are being found far from the shore, but there may still be laws against exportation.
I suggest you read these articles as they may help you: