Here is what MouseSavers.com advises when purchasing Disneyland Theme Park Passes:
Where NOT to Buy Disneyland Tickets
There are lots of legitimate ways to get discounted Disneyland tickets. Unfortunately, there are also some ways to get ripped off.
You should never buy partially used passes. For instance, someone may claim they bought a 4-day ticket and ended up using only 2 days of it, so they are reselling the remaining 2 days. Sounds good, right? The trouble is, there is absolutely no way to tell whether the pass does, in fact, have the number of days remaining that the seller claims, until you get to the gate and see if it works. If it doesn't, Disney will not be sympathetic, because Disney tickets are nontransferable. That means they can legally be used only by the original purchaser.
It is a very bad idea to buy Disney passes on eBay or Craigslist. Unfortunately both of these online resources are extremely popular with con artists. Recently scammers have even taken to presenting fake "invoices" or "receipts" that seem to be from legitimate businesses that supposedly sold them the tickets. They always have some sad story about buying nonrefundable tickets, and now they can't go, and they just want to get some of their money back. But the passes they sell you are partially or completely used up, or were never valid to begin with. Tickets can look perfectly new and real, yet be worthless. For example, they may be selling real Disney ticket stock shoplifted from supermarkets or other stores, but since the tickets were never paid for, they have not been validated and they have zero value.
There is no way to find out if the tickets you buy from a stranger (or even "a friend of a friend") are any good in advance of your trip. You are very likely to arrive at the gate and find out you have invalid passes. Here is a news story and another story about this type of scam.
However, there are legitimate, authorized ticket brokers that have contracts with Disney and sell NEW, UNUSED Disney passes at a discount!