What does Clean ESN, ready for activation on swappa or ebay mean?

I'm about to buy an iPhone that's new and also listed as Clean ESN, ready for activation. I know that means i can activate it with the carrier its locked to which is AT&T but can i use online unlocking service like iunlockpro or any another one to unlock it so that i can use it with any GSM? Or is this only for the carrier its locked to since it new?
thanks.

Added (1). the carrier is Sprint not AT&T

ESN stands for Electronic Serial Number. Like a unique VIN number for a car. Clean ESN means that the phone is activatable, that the phone has not been stolen or blacklisted by the cellphone company.

The bigger issue is that ATT and Sprint uses different cell networks. Kind of like, yes the radio does play AM and FM stations, but they are different frequencies. Unlocking an ATT phone will simply not work on Sprint or Verizon. It will however work with T-Mobile

Thanks but if the phone have SIM card tray and unlocked from its carrier, in this case from sprint does it matter which carrier i use it on. Plus none of those carriers are available where i live. There's only one company that provide mobile network with a GSM Sim in my country.