If you have serious doubt's about whether or not a site is legitimate, then you shouldn't even consider using it. Personally, I would never use an unlocker site. But I've never needed to. If the phone itself is legitimate, you should be able to get the carrier it's locked to to unlock it. I've never had a problem doing that. No need to pay for an unlock.
There's not much they can do with your IMEI anyway. An unlock code can only be issued once per number. If they think they've scammed a way to do something like sell multiple codes against an IMEI, well, they don't need your number for that. They could use anything.
Android since v1.0. Linux user since 2001.