I have just opened the box for activation.
It's brand new with box and all accessories.
It can't be both.

If you've opened the box, it isn't technically "new", but I do understand what you mean. Have you ever used the phone for personal use?
Selling it for 325 USD through BTC.
Someone offered you $250 and that was too low for you, but I think that might be the best genuine offer that you receive. You've probably already been contacted by many scammers offering you exactly the price you are looking for. I was thinking about offering close to $200, but I doubt you would be interested in that.
I'm wondering what happened that is causing you to want to sell this, because I took a peek around at big-box retailers and this seems to be how much you would pay for one brand new from them. I'm not trying to cause any issues, but there are many retail options for the same phone and the box hasn't been opened.
I bought it from straight talk 1 month back with 30 USD plan ( I will send this too ), It came around 300 with tax + plan. Next day i have seen a good offer for iPhone 7 and bought that too. So iPhone 6s is just opened for activation and kept as it is without any kind of use. I can lower the price to 300 USD because i have to pay for the shipping too.