[notwedtm]

While #1 is getting a contract, in "Mark's" case, he has a claim for Unjust Enrichment.

If you have a client that uses work you did, even if you have no contract, you can still sue for damages under Unjust Enrichment laws in the US.

[auctiontheory]

There's theory (taught at Yale Law), and then there's practice. While you may be theoretically correct, on a practical level, the lack of a contract makes getting paid much more difficult. Unfortunately, I can't bill the client for the time or stress of the legal case, and winning is never a certainty. Consider that the client is much larger, and may have lawyers on staff full-time.

[ams6110]

I'm guessing most freelance work is for pretty small amounts. Unless you are talking about a claim in excess of $10,000, it's not going to be worth pursuing, you will owe your lawyer more than that very quickly. If it's a local client you might have some luck in small claims court where you typically represent yourself.