Use Case Example:
I am outside a football game, walk up to a scalper ... he says tickets are 0.5 btc, i accept and transfer funds via cellphone in front of him, he checks his, sees its legit... then turns around to walk away without handing me tickets (no reverse transfer) ...
Now obvious anwser (dont buy from scalpers, check)
Other obvious anwser (you have same problem with cash, but in that instance I can kick his leg, knock him down, beat the shit out of him and get my cash back along with the tickets for trying to jack me; that doesnt work if i dont know his password on his phone after I get it back from him)
I would expect him to give me the ticket first because if I pay him I cannot get my money back, unless I beat the shit out of him and make him transfer it back to my address
So simple solution:
1. Get the item first
2. Beat the shit out of him