The seller just won't sell me an item.

Everyone buying has ran into this issue at some point along the way. The it's not for sale guy who despite having that car sitting in the yard ten years now that's half eaten away still has dreams of flying down the road in an imaginary mint condition restored version of the car you're trying to buy.

In these cases there's often not much to be done aside from biting your tongue and walking away mumbling to yourself.

You can try to politely point out that you have cash in hand and the project would be something nice for you and your son to build on the weekends in your garage together but when you encounter these sellers or lack there of there's really not much hope in persuading them to sell that imaginary gem they've dreamed up in their heads while watching shows like Gas Monkey and Pimp My Ride.