The last castle I tried to buy was privately 'negotiated' and in that case it became pretty clear that he was just trying to inflate the price by playing multiple "people" against each other in a silent bidding process in which I suspect I was the only living breathing person actually bidding.
While I recognize that even with public bidding you could do the same by making multiple accounts here, or having friends bid an item up, at least the process is somewhat more transparent.
I'm not suggesting that you are shady, but I am unequivocally saying that others are.
Could you provide a link to the thread where that castle was sold? I generally consider my Google-foo to be quite good, but sadly I am unable to find such an example on these forums. The most recent castle sold on this shard seems to have been for 1B - but that thread is one in which only the seller acknowledges the sale, so I take it with the requisite grain of salt, this being the internet after all.
In any event, I don't think we will make a deal, I'm not willing to pay "extra" for Tram vs Fel, given then I'd prefer Fel, so we're off by >1B in valuation.
I wish you good luck on the sale.