You are buying the dog from someone the owner?
Not rehoming through a rescue?
Firstly, that's a disaters waiting to happen. Yes, I am sure it works out sometimes, but generally it doesnt. The owners often lie to rescue (hence why rescues have an assessment period) and the new family arent prepared or experienced enough to deal with it.
My last foster had been rehome twice privately. Thankfully he last owner had the sense surrender him to a rescues and not sell on again.
Secondly, you arent rehoming. It's not a rehoming charge.
I adopted one of my foster dogs and she was £300. I was happy to pay it. I get so much support, with my adopted dog and with the foster dogs I have, I think it's well worth the money.
A decent rescue wouldnt let the owner negotiate and sell the dog. Because it wont have been assessed. I have known it happen a litter of puppies stay with the family. The owner of the dam died. The owners daughter was looking after the dam and pups and contact the rescue to rehome them as she wasnt an experienced breeder/seller herself.
But the rescue had people visit several times a week to provide support and better the homes for the puppies.
This would not happen with an 18 month old dog.
Dont kid yourself it's a rehoming fee. You are buying a dog