I'm happy to agree that "all sellers in a market perfectly coordinate on price" is functionally equivalent to "there is only one seller in the market", and that this qualifies as a monopoly. My comment doesn't hinge on that obvious point. (Indeed I said "alternatives from another seller", not just "alternatives", and it's pretty clear that nothing changes if all the sellers perfectly coordinate as one.)

Your explanation doesn't save your example. It will always be the case that, for any given buyer in a non-commodity market, one choice is better than all the other. (If the products are not identical, one will essentially always be at least a bit better than the others for some reason.) However, this doesn't approach a monopoly except as the alternatives become arbitrarily bad in comparison (i.e., do not function as substitutes). Yes, in this limit, it's a monopoly, but this is almost never the case.

Your remaining comments imply that anything sold at auction is a monopoly and not a free market. You also incorrectly ascribe this to lack of mobility:

> In effect, each unique location is an auction, with a different group of buyers interested (at different prices) each time. This only happens because land doesn't move.

Art, used cars, items on eBay, etc., are all products that move (unlike land) yet "each...is an auction, with a different group of buyers interested (at different prices) each time". Again: the key mistake is that you are conflating all non-commodities as monopolies.