It could not lawfully be sold
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How it happens
A fossil picked up on public land years ago sits in a drawer. The collection gets sold or inherited and listed. Selling or offering to sell is prohibited where the person knew or should have known the resource came from federal land.
Should have known is the operative standard
Not did know. An absence of provenance on a specimen of a type and from a region strongly associated with federal land is not a shield; it is much closer to the problem itself.
There is a date exemption, and it is narrow
Nothing in the prohibited acts subsection applies to a paleontological resource in the lawful possession of a person before 30 March 2009. Lawful possession is the condition, so it does not launder something taken unlawfully before then.
Casual collecting does not help here
Even where it applied, it covered common invertebrate and plant fossils for non-commercial personal use. A specimen lawfully collected under it never became salable, because the limitation was on the collecting.
Vertebrates have no personal-use story at all
For a shark tooth, a megalodon tooth, a fossil fish or any bone, there is no casual collecting route to point at. The land is the only answer, and it either exists in writing or it does not.
Penalties scale with value
Up to five years, or two years where the sum of commercial and paleontological value plus the cost of restoration and repair does not exceed $500. A second or subsequent violation by the same person can double the penalty.
Keeping it is not the same as selling it
The sale and the labeling offenses are about commerce and records. For a specimen already held with no provenance, not listing it is a real and sufficient answer, and it is usually the right one.