If I buy an iPhone 8 from someone that s been paid full price, will I owe them any money?

She said it has been paid full price and can go to any carrier

Sounds like "she" is selling an unlocked iPhone 8. If that is the case, then yes you could go to any carrier & no, you wouldn't owe money.

Well, you'd owe whatever money you said you'd pay for it to the seller, but you shouldn't owe anything extra to a mobile carrier.