Legit Second hand phones shops online?

My daughter would like a new phone for christmas and said she would prefer a second hand one as is much much cheaper! Does anyone know any good legit online second hand phone shops? We're in the UK and would be looking to get an iPhone 7. I have looked on 4Gadgets but is out of stock there.
Also does anyone know if second-handphones.com is legit?

Buy a new phone or try to fix my current one?

A few weeks ago I dropped my iPhone 6s (owned for less than 9 months) in the toilet at a buddy's house. I dried it off as quick as I could but some damage was already done. At this current moment the volume up button and volume down button both turn down the volume. My service goes in and out and my battery drains faster than usual. Now I'm just trying to figure out what to do. Is it possible to buy the same phone (paid for in full)? How would that affect my current contract/plan? I would be willing to pay for a new phone if the hassle of fixing my current one is not worth it.

Somethings going on with text on my phone?

So when I try to type something down on notes on my iPhone, it works but whenever the text reached the end of the screen it just displays the word I typed and not the sentence. How do I fix this?

How much are the Iphone 7 Plus's selling in the UK for?

I'm visiting the UK and I would like to know what the prices of the Iphone 7 Plus is?

Added (1). What is a reasonable price to buy them in the UK?

Added (2). How much are the Iphone 7 Plus 32 GB?

Added (3). What do the phone stores like Carphone Warehouse sell them for?

Iphone 7 won't charge unless I hold my phone at a certain angle?

No, it's not the charger and yes I have checked for lint. I bent a paper clip and dug around in the port and only a bit came out. Is the paper clip not effective and there actually is lint in there? How could I fix this if it doesn't have to do with lint or the charger? (besides going to the apple store)