Hmm I heard about that but is there any consequences to using the unlock from ebay? And how does the unlock from ebay exactly works?
The "consequence" is that buyers can't tell if it was a factory unlocked phone or not. As such, they worry, and pine, and stress because they think some day the carriers might re-lock the phone (and despite this belief, there hasn't been one documented case of an iPhone 4S or 5 (the only phones new enough to use the newfound service) being relocked). The "re-locked" phones people reference were in fact probably just jailbroken unlocked phones that got updated by stupid end users after being told not to.
AT&T, Verizon, etc. would be crazy to try and re-lock a phone that may have been legitimately unlocked. There is NO way for them to tell which is which once the phone is on Apple's list, sorry to burst the conspiracy.
The unlock itself works like this: They use the carriers unlock site to unlock your phone through Apple. They gain access the same way legitimate users do. You can use a form and AT&T, or use them for a few bucks. It's THE SAME SYSTEM. Sure it's a little shady that they do it without paperwork, but it is 100% going unnoticed by the carriers and if it gets shut down in the future they won't have any way of going back to "re-lock" phones (nor would they, it'd be a PR disaster even if they could figure out which was which (they can't)).