When you buy a Bitcoin you are not buying equity in the Bitcoin environment. Wonder what model BoA valuers have in mind for this. It weirds me out.
Almost certainly they are using a Commodity Futures Contract model that is used for precious metals, oil/petroleum, wheat and so on, on places like the Chicago Board of Trade or the London Metals Exchange. With those, there is a finite new supply of "product" and for the vast majority of people dealing with them, there is no physical product - the contracts being traded are for future delivery of a specific quantity of a specific product on a specific date. The product will be delivered to a specified place, and it is the responsibility of the person receiving the product (the contract owner as of the contract maturity date/product delivery date) to move it from that delivery point to a storage location of their choice.
As such, the traders who buy and sell these contracts do not want to hold them until maturity, because they have no storage space, so they buy the contracts (for example) 12 months before the delivery date for a specified price, and sell them before maturity, hopefully for a profit.
With bitcoins, there are no physical deliverables, but in every other important sense they resemble these Commodities Futures contracts.
If they became widely accepted and centrally traded, with a central body guaranteeing transaction integrity (basic ESCROW systems would probably be the starting point for that), they would almost certainly be traded as a "standard" currency on FX markets (exchanging Bitcoins for US Dollars, Euros, Japanese Yen, etc.), but without that central body guaranteeing the transactions, they would probably be traded as Commodities Futures.
The "problem" with both of those routes is that there is heavy auditing on every stage of every transaction, so the anonymity aspect of Bitcoin goes right out of the window. The parties involved in any given transaction would be known and recorded, and even if those are brokers acting on behalf of the real Bitcoin owners, the brokers would still need to have records showing who the real owner is.