What aspects of selling a mobile phone are there?

Hi, thanks for any answers.

I am on a mobile contract (in the UK) that I have just renewed with an upgrade phone. I have moved from an old phone to this new one, bypassing the use of the last upgrade they sent.

I’m keen to declutter and have been told the phone is worth selling.

My question is, what state is the phone in with regard to network access?

Is it the case then, that to others, this phone is “locked” and that in my advert on Ebay I should say “For sale….. phone in locked condition” etc… just to be up front about it’s condition, as it is new, but ready prepped for me as a customer of theirs.

How much is it worth, Sony K800i?