It's your money and you are free to toss good money after bad as much as you wish. For most phones in the USA locked to a carrier, the carrier will require that you be a customer, or have been a former customer who left with an account in good standing in order to process an unlock. Beware that many such iPhones sold end up with people posting here that they are blocked from use by Activation Lock and they have no way at all of contacting the former owner to see about getting that removed. In that case, you just bought an expensive door stop.
If you want a trouble free iPhone experience, then buy one from Apple or an Apple Authorized reseller in the country you live in or plan to use it in. Otherwise, pay your money, roll the dice, and take your chances. These forums are littered with posts of people who looked to save a buck the same way you are, only to ultimately end up with a device that is utterly useless to them (i.e. it is activation locked and so useless, carrier locked and the carrier will not unlock it, hacked to work as unlocked only to relock the next time you try to upgrade it, has an IMEI that is blacklisted in the country they want to use it or on the network they want to use, they experience a problem but cannot get warranty service because it was not originally sold in the country they are in, they experience a problem and cannot even pay for out of warranty service because the device has been worked on by non-Apple service centers, etc, etc, etc).