You'll find almost as many answers as there are collectors.

My collection is entirely self collected, all from a stretch of reef
about seventeen miles long, and most were taken fresh dead, a lot from
octopus middens.

Saturday I found a very large very old very beat-up S gigas with a dark
royal purple aperture. She's still on the bottom, not because they are
protected here, but because they are rare here and she ought to survive.
We chatted for a bit, then she went on her way as did I.

Exceptional specimens of common shells might meet a different fate.

I have never bought, traded, or sold a shell, but have given away
hundreds. Maybe one of these days I'll sell some of my spondylus. The
rest aren't worth much anyway.

Do your own thing, whatever satisfies you, and don't let "experts" tell
ya how to collect or what to collect.

m