Buying from someone you don't know, on the street, is not a good idea.

There are many counterfeits, copies and scams out there.

Occasionally, the parks will offer "second day free" tickets.  They may be resold to naive tourists, who are then turned away at the gate.  It happens because they are not the same ones who bought the original tickets.  Remember, the parks will check your I.D.

Be careful of handling fees.  Some places that offer discounts, charge a large handling fee.  That eliminates your discount.  Always find out the total price. including all taxes and fees.  You'll probably be better off just buying them from the park.  Many ticket agencies charge an excessive "restocking fee" if you return the tickets.  Don't ever use these places if  there is a chance that you won't be using the tickets.

It's not a scam, but many discount tickets are sold with the stipulation that you attend a sales presentation of a certain length. Although you will save money, be sure that you understand the commitment necessary.  Consider whether the discount is worth the time or if you even have an interest in the product that they are pushing

Check the "small print" for restrictions.  Many tickets are valid only on certain days or during certain time periods. 

Again, if it sounds too good to be true it probably is.  View any special deals with caution and scepticism.