Could it all just be semantics? Claiming that you own something is a bit of a farce in a feudalistic society, specifically land. You will still pay the count either way to have the land to work with.
The only thing that would change is the name on the deed contract, people that work and develop the land within the counts domain just have responsibility to pay the count for the land in increments.The same would happen if he owned all the land and then rented everything out via a less automated contract system that bypasses the land table.
It is actually quite advantageous to rent the land rather than owning the land. The only thing that renting does is not increase fame level as quickly. This comes with pro's and Con's. But lets say you rent and are free to develop land, what then happens when they then want to rent out parts of what they rent from the count. Again it's just semantics and adding a second step than just leasing land off the table..
Another thing that this changes is what is actually being traded, the kingdom's (county's) land or the player's (player that is a count's) land rights.
The only thing that this "games" is that the count would prevent the fame level of people that rent rather than buy from the land table as much fame, which would make people within his domain less of a risk of fighting him internally with a legal Cassus belli.
Maybe a few people would rather rent things from the count than lease things from the land table. That's all about perspective, would you like to slow the rise in fame/notoriety? Or do you want to increase fame, and increase the cost of your death so you can achieve positions of power such as possibly a mayor? But renting or leasing from the count is semantics. You never technically own land in this game, unless you are king.
Unless I am mistaken, not many counts would do a one and done fee for land, only special circumstances. Most counts would use a yearly fee or some other method of time through testing to get money in return to pay the count. Then the count pays the percentage to the duke, the duke then pays a percentage to the king. That's the most straight forward way of dealing with taxation is through land. The other form of taxes can come way of assisting trades and confirming routes to be guarded.
Income, gains, trade, and other ways of gaining money would be hard to audit or follow, counts will have an idea of how much resource is coming out from a given area, but they wouldn't be precise, which may cause a problem.
Renewal would likely happen monthly to account for a change, of some kind to gauge the value of the land accordingly. You possibly can luck out for a month before the count catches on that you struck gold within your land. But once he notices that.... That there is a time limit.
As a mayor things are a bit different you have the ability to zone once you get a town hall up and running and meet the requirement. Essentially you direct the flow on a micro-level rather than the macro that count and up focus on. If you have a goal for the settlement you aid and assist, Even as a mayor you may even have a land table of your own to sell and pars out your land to new people. That's also up to the deal you made with the count for the land you own within his/her domain. If you are savvy enough you can run your mayoral settlement much like a count would, while also having access to zoning and planning tools. Though that may be much harder, but if you own 70 of the 81 in a capital, you would come close to achieving a mini-county.