IANAL, but I think you'd have a hard time convincing a judge of that math.
The standard measure of damages in contract is expectancy, that is the breaching party has to put the non-breaching party in as good a position as he would have been had he not breached.
Had the seller not breached the buyer would have had tickets with a fair market value of $6000 (at the time of breach) and been out the $900 he paid for the tickets. After the breach the buyer got back the $900, but is still $5100 worse off than if the seller had fulfilled his part of the bargain. Those are the expectancy damages he suffered.
Given that going to this specific game is a unique experience that can't be duplicated a court could order specific performance (i.e. that the tickets be handed over) as tacon suggests below. But: 1) I don't think small claims courts can order specific performance, 2) you probably couldn't get before a small claims court judge in time, and 3) if the seller didn't actually have the ticket then he couldn't be ordered to turn it over. So monetary damages are more likely.
I guess I don't really follow why the buyer is entitled to appreciation on something that was never in their possession.
If the tickets had instead declined in value before the seller backed out then wouldn't the buyer still get back all their money? I assume it would be unacceptable to refund them just the then-current FMV, so why is it different when the tickets appreciate in value?
The buyer owns the tickets from the moment the purchase is complete, whether or not he has received them.
If the tickets went down in value in the interim time, then the most expedient thing for the seller to do to cure the problem would be to buy new tickets at the now-reduced price and hand them over to the buyer. You are absolutely right, the buyer can't demand his money back if the tickets go down in value -- he has to accept tickets that are fundamentally the same in value as the ones he was supposed to get.
Now, if for whatever reason the seller couldn't replace the tickets even though the price has dropped (unlikely -- that's pretty much the definition of a price, is that the item is available in exchange for money), then he might be able to ask for his money back instead.
Obviously they could try a cause of action for lost opportunity to profit from the transaction from legal standpoint, but it's much harder to claim losses from hypothetical things that never happened than to unwind an actual transaction.
What if Kobe was injured and the price of the tickets plummeted for example? It's a stretch to argue that the buyer lost something tangible by relying on the sellers performance of the contract.