This is a common scam, and something Disney is fighting against. Here's how it goes:
Scammer buys a 5-day ticket, often in e-form (computer print-out with bar code). They use it however much they want.
They offer the ticket up at a huge discount.
The buyer uses a few days, and then sends it back (this may not always happen, depending on the scam).
Then, if there are days left, the scammer sells/rents the tickets again, again at a huge discount over
Disneyland-bought tickets, and the scammer has made $$$ without doing anything but sending out the tickets.
Another common variance is the scammer printing an e-ticket out multiple times, and selling the copies. People don't realize the barcode is person-specific, trust the seller that its "good", and get to the gates to find out the original print-out was already used, so their e-ticket isn't worth anything. Then the scammer has made even more money off of just the price of the original e-ticket.
So, why won't renting a ticket work? Disney takes pictures (usually on the first day of use). So, unless you look like the first person to use the ticket, you're out of luck when your face doesn't match their picture. Also, if they suspect some sort of nefariace activity, they may pull you aside and ask you questions about it (when it was bought, cash or credit card for the initial purchase, etc). And names are printed on most tickets now. Scammers renting and reselling tickets is a HUGE reason Americans can only get 5-day tickets now, instead of being able to add days on like we used to do.