Unless you personally know the seller, or they are on eBay with very high positive feedback as a seller I would stay away from them.
Because the only things encoded on the magnetic stripe (and bar code) are what type of ticket it is and the serial number of the ticket, you have to have access to a Disney Automated Ticketing System computer to read the ticket and see what actually remains on the ticket.
And if the ticket is partially used, unless it clearly states it has the No Expire Option it is normally only valid for 14 days from the first time it went through a turnstile. Additionally if it has been used it will have a finger scan tied to it. There are ways to tell if you are the legitimte owner, and if you are not the best you will find is you will be allowed to purchase full-price tickets on the spot.
Note that all tickets state on them they are not transferable, and additionally it is a violation of Florida law to sell partially used theme park tickets.