iPhone camera shutter sound

There is no such thing as an authorized seller on eBay.

You bought a gray market phone that undoubtedly was originally sold in Japan or South Korea. There is nothing you can do about it if that's the case other than to return it and buy one from an actual authorized source instead.

Check the model number in Settings>General>About>Model.

That is not the complete model number.

Look in Setting>General>About>Model. What is the COMPLETE model number as listed there?

If it is from a country where that is restricted, nothing can change it. No software or firmware update will change that behavior.

If the phone was intended for sale in the Japanese market, you can't silence the shutter sound. However, you can use a camera app such as Manner Camera. Or, you can simply position your finger over the speaker as you take the picture. It mutes it pretty well.

I've found that 99% of the time, if I'm worried that people will hear the shutter sound, it means I'm probably taking a picture at a time or place I probably shouldn't.

Did you buy it from Apple or an authorized reseller? Or was it a private sale? If the latter, it may be one intended for sale in a different country.