This is a fairly simple suggestion;
Basically, the option to buy/sell land in the same was in the main grid, but on a private sim. You how much rent (if any) must be paid on the land. So you could parcel off plots of 1024sq metres, and charge L$15000 each for them, they up a user's tier as normal (if they already own land), and they then pay $50 rent a week for the extra benefit of it being a private sim.
However, the monthly USD value of the new land-owner's tier is deducted from what the sim-owner must pay, down to some minimum (say you sold all the land in your private sim, you'd still have to pay L$20 a month to own it).
Ownership is still much as it is, you still can shape the land, access all areas and kick people off the land if you so wish. But if they are paying for it, then it takes 72 hours or something to do, so they've a warning/chance to pack-up before it happens. If you do this, then the tier cost that the user offsets from you depends on how far into the month it is.
The idea is to make managing land sales in private sims much easier, but still have the options to add value to it; ie L$ rent, deducted automatically each period (say a week). Buyer is told of it before buying, and must agree to any change in the rent, if rent changes and they decline the change, they are removed in 72 hours as above or something.