Well, first of all, there are the physical Bitcoins for one thing, which can be collected and traded neatly.
If you wanted to trade such historical Bitcoins, you'd probably need to follow some rules. For example, if you were interested in having pure Bitcoins and as close to the original transaction as possible:
- Any Bitcoin that is a part of a given transaction, or a specific input is 100% pure and 0 removed.
- Any Bitcoin coming from a transaction of purity p and removed status r is p% pure and r+1 removed.
- Any Bitcoin coming from a set of transactions, one with purity p and quantity q1 and any others of total purity 0 and quantity q2, would be p*q1/(q1+q2) pure and again, r+1 removed.
- You also have to take into considerations blocks that collect transaction fees for all of this, but I'd consider them to be 0% pure.
So, in the end you would be collecting your beloved Bitcoins to one or more addresses you
own and not spend them, you'd value transactions that are closer to the source and less diluted. If you'd want to flaunt your collection, you'd just show the unspent Tx list and sign some message with your private key. If you'd want to sell any collected Bitcoins, you'd just create a transaction from one of the unspent transactions, and hopefully not spend any coins on transaction fees, as that would make them more and more scarce.
I hope that covers everything. It was an interesting food for thought.