Hi Liang!
I have seen many of those offers on the internet as well, and have only every taken a chance on purchasing tickets from a reseller just once. In that case, I had purchased three day tickets with the free upgrade. When the tickets were mailed to me, they were 5 day tickets (this was stamped on the back).
Since the experience made me quite nervous (All sorts of questions ran through my brain: Would the tickets get here in time? What if they weren't valid when I got to the gate?), I opted to purchase tickets via Disney after that. I wasn't sure that saving just a few dollars would be worth the worry to me.
I'm not sure how nervous the idea of purchasing makes you - only you can be the judge of that, but my best advice is to be sure and research any ticket reseller before you purchase. I, personally, do not trust auction sites or any tickets which are advertised as having valid park days still on them.
Also, if you are thinking of trying out a
special offer like the current Free Dining promotion, be aware that these packages require a ticket purchase along with your hotel stay. In this case, you may save more with one of these deals rather than trying to save on components- but be sure to check out prices since this varies based on which hotel and what time of year you're planning on visiting.
Hope this helps- and I hope you find a great deal.