The research you describe is very hard to measure. You can visit a land and see not a single soul on it for hours.
Without any data you are making a too big risk. Also the effort the current owner does also affects to ROI ( advertising/events).
I have bought a few lands and the owners did provide all the info. Even then its hard to trust the sellers.
It is so easy to manipulate the ROI. For example hunt all day on your land. Buy items from a shopkeeper (with tax) or let others hunt/mine on your cost.
But messi is right, no one is obligated to provide the data. In most cases i would advise to turn the deal down unless you are confident to buy it.
I spoke with Adm a few year ago, about this dome. He doesnt advertise since its a dome. The returns were nearly half what a CLD gives you.
Thats why we turned it down since we found out that all players have their own habits (favorite hunting/mining spot).
In the ends its a free market, he can ask what he wants but he didnt have any luck in the past years he tried to sell it.