I'm interested in this but is there anything wrong with it?
No there's nothing wrong with it at all. It's brand new, never been used. 
And will I be able to use it on any phone service?
Yes it can be used on all carriers. 
I would like to buy this for 540 today. 
That's not what they are going for. They are going for 850 and I'm selling them for 780 to sell them faster.
I cannot pay that much. I have seen most closer to 600, which is more in my price range. 
Check again, most of them are selling for 850, this is the 128 GB version not the lower ones. I can go down to 750 but that's it. 
How about 720?
740 and we have a deal.