OP, I think you're being fair. You've got good reason to offer the price you did, and you understand and accept that the seller may not accept your offer.

I disagree that anyone is a crook for asking a higher price than they'd actually accept, even by 2x. Nothing at all wrong with having a price in mind that you will not go below, while at the same time putting a few bucks in your pocket just by asking. The buyer is under zero obligation to pay the higher price, if they find it objectionable.

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617 swapped Toyota Pickup, 22-24 MPG, 50k miles on swap