Honestly, people do have regions they not need. But with the new rule alt usage is forbidden and only 30 max regions.
The problem is the low supply.
Many houses get unregioned so people can have private islands or redo them.
Many players get a house and leave the server after the summer. 60 days later the house gets up for sale again. So 2 months of a quit market. This just happens.
Want to fix the market indirectly? Make new players stay more often and longer.
Want to fix it directly? Create more houses. Nice ones, empty ones, anything will do. Make them priced sharp. So 250k for a plot worth 270k. Have some trustable realtors sell the regions to newer players to prevent hoarders and tadaa. New houses on the market.
We could just tax everyone bank and balance every month and the more you have the more you get taxed so you are less likely to buy and hoard plots because you have to worry about losing money to taxes
Taxes won't do a thing. If everyone gets taxed you just make money worth a bit more on its own.
Taxes is a system by the goverment to split money more evenly over the people, and to pay for the services of the goverment like roads, schools etc.
Taxes are not used to decrease the amount of money in the economy. Using a percentage a plot will be one of the 2 things:
1. Useless. 10k a plot a month for rich people. Nope. Also complex. What if you can't pay it anymore or you are temporarily inactive.
2. Unfair. 100k a plot will also punish the poor. And a lit more then the rich.
Like dear Shadow described.
My points of view