I'm just chiming in to echo what some of the other intelligent folks have already said.  The "as a business" requirement is a threshold one.  Unless you are acting "as a business," you can't be a Money Services Business. Simply failing to incorporate or organize as a corporation, limited liability company, etc., is not dispositive.  It is only one factor, actually, in the "facts and circumstances" test that determines whether you can be fairly characterized as a business.

OP, you're right in characterizing this as a smell test, or a "I'll know it when I see it" test.  It might be frustrating, but FinCEN determined it was better than trying to give another paragraph of enumerated monetary thresholds. I'll leave up to debate the question of whether a list of monetary maximums would be more effective.

Most of the lawyers dealing with this issue are pure transactional folk, so, as a lawyer who actually litigates, maybe I can bring a more concrete perspective to this.  To be sure, there is no single circumstance or fact that determines whether you are a business.  Instead, you should think about the only thing that ultimately matters: a courtroom.  If you had to describe your trading activity to a jury, what characteristics of your activity would you want them to see?  You'd probably want them to see that you only trade infrequently; that you typically trade with a single counterparty or a small group of counterparties; that those counterparties are registered MSBs; that you don't advertise; that you don't have any website or brick and mortar location; that you don't claim any deductions against any profit on your tax returns; that you don't have any investors; that you don't have employees or regular assistants ...that sort of thing.

Thinking like a litigator helps avoid litigation!