I think it’s worth saying that obviously a fair deal should be fair to both parties.
Any watch has a range of values, and while condition is the most significant part, the status of the seller also plays a part when I’m valuing a watch.
If you’re buying from a dealer, you (should) have the convenience of being able to pay by card, a warranty, a return period, a recent service, confidence in their sales history, the ability to view good photos, the convenience of having it shipped out to you so you don’t even have to leave your sofa as well as the fact that the dealer will price it.
When buying privately in an extreme example you often have none of this. You likely pay cash, sold as is with no returns, no warranty, you’re relying on your own knowledge to satisfy yourself everything is correct, the photos are often terrible and it’s often the buyer who has to come up with a price and then drive to the seller to pay and collect at a time to suit the seller.
It goes without saying that people should absolutely not low ball or try and rip naive sellers off, but i don’t expect to be paying dealer prices if I’m the one putting all the effort in and taking all the risk.