Is environmentally-driven purchasing of coal mines a good idea? Certainly environmental non-profits won't have enough funds to do it on a large scale. Governments have, but they are corrupt. There's a huge risk that they'll just buy up those mines under the pretext of climate change and after the reserves deplete elsewhere and the prices soar, it'll just be one colossal fossil fuel cartel that would dwarf OPEC immensely
No this won't work, because there's so much coal reserves, unless you buy all of them.
With oil it might just work. Buy an oilfield and promise to not utilize it in any manner. However, the fossil fuel industry is probably capable of finding more oilfields than there is room for carbon dioxide in the atmosphere, and electric car transition is rapidly progressing, so I wouldn't be surprised if we naturally end up with unused oilfields. So the plan to buy oilfields could only work if there are more buyers for oilfields to be left unutilized than what oil would be left unutilized naturally.
With natural gas it might work. Natural gas will be used as transition adjustable power in electricity grids (because we don't have enough hydropower resources), to be replaced later with hydrogen. If you buy natural gas fields and leave them unused, it might accelerate the transition towards hydrogen. Yet, I wouldn't be surprised if the hydrogen transition accelerates so much that natural gas would be left unused naturally too. So not certain it will work, like with oil.
With coal no way is it going to work. The problem is, we have massive massive coal reserves, so much that using them all would warm the planet at least 10 degrees Celsius if not more. No way would all of them be utilized naturally. So your plan would work only if you purchased every single coal mine on this planet, and would continue buying them at a great rate as new mines are found.
A simpler way, much cheaper, would be to just buy carbon dioxide permits and promise to leave them unused. However, if enough many people start doing this, the policymakers deciding the number of carbon dioxide permits will increase the number of future permits, knowing that part of them will be left unused by intentional buyers, so the effect might be negated.
Another simpler way, much cheaper, is to buy forest and create biochar from the trees. No need to find all that money for a single oilfield, gasfield or coal mine.
Also a very important (if not the most important) way to prevent dangerous climate change is to vote in every election for a candidate favoring the environment. Also you should change all home appliances to the most energy efficient appliances possible, and switch to an electric car. All of them are cheaper than buying coal mines. Any unused money should be invested in companies that do good for the environment.