Im too afraid to buy from those websites. What if they send you a refurbished phone? How do you make sure the phone is BRAND-NEW and not fcked up inside? Why do these websites sell the phones for cheap? I mean, there has to be a catch...right?
Can't speak for Letstalk, don't know them.
Wirefly sells for less the same way Amazon does - major strings attached for early termination.
Wirefly has a support account on our forums, not many vendors are allowed that. You get a problem with them, you let them know and ok to do it in a public thread for all to see if they treat you right (you'll exchange account details privately though).
Not pimping for them, just being fair letting you know your options.
Besides, most phones have some sort of service menu you can get into and check its status. Don't know it for the SGS3, sorry.
The most important thing about Wirefly is you have to read the contract details carefully before you go with them. Some do and run away, others do and come out happy.
You have the attention of a million plus members strong forum here, and they're ok with that. It's therefore generally win-win for everyone.
BTW, androidforums.com doesn't make a dime on them, so I have no conflict of interest here. Members first!