Depends on a lot of factors, first, there will be no more new coal stoves available this year, the entire production of new stoves is sold out untill next spring..
So if that Mark I is in really good shape, nice paint, clean glass, very little use, then it probably is worth what a new one would be worth, maybe a bit more if someone can't afford oil or propane this year.
So Around $1500 if it's really nice.
If it has several seasons of use, the paint is faded, and has some surface rust showing, the glass is cloudy or opaque, but the grates function well and it is just cosmeticly not 'like new' then I'd say around $900-$1000.
If the stove is really rusty, has been outdoors, needs sandblasting and painting, needs new glass and the grates show a missing tooth or two, or are sagging from full ashpans and hot fires,, if the structure of the stove is intact and usable,, I'd say $400-$500.
Any values can go up if the buyer is desperate to get a coal stove, or go be lower if the seller doesn't care or is not aware of the current buyer's market for coal appliances..
Even if a person pays top dollar, say $1500 and buys two tons of coal for $500, the savings in oil will just about equal the money spent. If you pay $1000 for the stove, it will pay for itself this season. This is based on 180-200 gallons per ton of coal x $4.00 per gallon = ~ $1600.
That's my $.02 worth.. there are still great bargains out there, as well as a few almost new stoves looking for homes. I think there is an as-new condition Mark I in the 'for sale items' forum..