Where you found it decides most of it
On this page11
Public lands and Indian lands
ARPA applies here specifically. Excavating, removing, damaging or defacing an archaeological resource is prohibited without a permit, and the statute names those two categories of land.
Private land with permission
A different position entirely for archaeological material, though state law, burial laws and local ordinances still apply. Permission from the landowner is the beginning of the question rather than the end.
National parks
Generally the strictest. Collecting of almost anything is prohibited in most units, including shells, rocks and driftwood, and the rules are park-specific rather than uniform.
State parks and beaches
Vary widely, and often by season and by whether the shell was occupied. Several states regulate the collection of live shells or set quantity limits on sand.
Which is why "I found it on a beach" is not an answer
The beach belongs to somebody and is governed by something. Knowing which authority is the actual first step and it is usually posted at the access point.
And the commerce question is separate again
ARPA subsection (c) reaches selling, purchasing, exchanging or transporting in interstate commerce anything obtained in violation of state or local law. A lawful find can become an unlawful sale.
Record where and when
For your own sake. A find with no provenance is a find you cannot defend, cannot date and cannot sell, and the record costs a line on a label at the moment you pick it up.
When in doubt, photograph and leave it
A photograph of an arrowhead in situ is more scientifically valuable than the arrowhead in a drawer, and it carries none of these questions.
Boundaries are rarely marked where you are standing
Beaches, river banks and desert washes cross jurisdictions without a sign. Knowing roughly which agency manages the area before setting out is the practical version of this page.
Tidal land is its own question
In many states the land below the high water mark is public even where the beach above it is private, which means the same walk can cross a boundary twice without anybody noticing.