Collector, or dealer
Updated
Almost every dealer started as a collector, and the transition is the most consequential one in this craft.
The moment it changes
Not when you incorporate. When you start acquiring material in order to sell it.
Selling duplicates from your own collection is what collectors have always done. Buying a flat because it will resell is a business.
The line is intent, and intent is what several of the rules turn on.
What changes legally
The big one: the BLM personal-use allowance is noncommercial. Selling material collected under it requires a permit or a claim.
So collecting that was lawful for your collection is not automatically lawful for your stock, and the change of purpose is retroactive in effect even though the collecting already happened.
Plus the ordinary business items: registration, sales tax, and sales tax in the states where you attend shows.
And insurance, because a household policy generally excludes business use.
What changes practically
Volume, and with it storage and the backlog.
Documentation becomes commercial rather than personal: a buyer relies on it.
Grading has to be consistent, because you are now pricing against yourself repeatedly.
And your reputation becomes an asset with a value, in a small market where everybody talks.
Doing both
People often do, and the thing that makes it work is a hard line between the collection and the stock.
Separate catalog, separate storage, separate numbers. A specimen moves between them by a recorded decision, not by drifting.
Without that line, the collection quietly becomes stock, the best pieces get sold in a thin month, and the thing you were building stops existing.