Getting an attorney sounds like a good idea, but being an attorney myself, for the amount of money we're talking here, you'd be likely throwing good money after bad.
At best, the most damages we're talking about would be loss of value. You do not get emotional distress or punitive damages in a contract dispute. In this case, he'd get the value of 300 points -- which is $4500 on the high end and about $3000 on the low end. I still don't think we've seen exactly when these points were to expire -- but there's a good chance that they would have been almost impossible to use anyways if they expired prior to Dec. 1, 2016, given that damn near everything is booked in WDW.
In any case, even at $15 a point, it will cost this person at least $3000 to $5000 to hire an attorney to try and sue the person. Even if successful in court, you still have to actually COLLECT the money from the defendant. This is not an easy task if the defendant fails to cooperate. You'd basically become a bill collector -- and who wants to do that??? You could always take them to small claims court yourself -- but you'd likely have to file in the defendant's home state or Florida, and if that isn't where you live, you will likely need to hire an attorney.
Again -- I think the best way to handle this is to file a police report for grand theft. The buyer should have the seller's home address, so this would be quite easy for the police to contact the person. This could also light a fire under the seller's butt to actually negotiate in good faith.
Has the seller even offered any type of refund?
At the end of the day -- the two most likely outcomes are get a full refund at some point in the distant future -- or just accept the loss of the 300 points and move forward. I would have thought the seller would have agreed to a partial refund of some sort, but since you cannot force that outcome, there's really only the two other options.