No problem. There would be a market-place like eBay where all such properties and their values are listed, and at given times a sale can happen for it, prospective buyers' bids are listed in advance. The best price wins the property. An Owner can keep the property by declaring it at a high enough price as the auction starts. If they lose the bid anyway and still want to keep it they must raise the price even higher at the end of the auction and pay additional penalties for guessing wrong in the first place. An owner can lower the price to if they aim to sell or think they can get away with a lower property tax. This is the way it worked in the aforementioned novels anyway...though for land properties, no mention was made for intellectual properties.

IIRC, there was a study done which concluded that 14 years was the more or less optimum time period for most intellectual property as a protected monopoly and such. Assuming this is true, I would move the number to 10 years and place the property on a yearly auction/property tax. After which, the owner must declare a sale price, and pays a small proportional property tax for every year it is not sold. If it the property is crap it will be put in public domain forthwith by declaring $0 in value for year 11 (this is the default value at this year), which is obviously not taxed either. If the Intellectual property is $hiny the owner will declare a high value and hope noone buys the property, AND hope the guessed right so they can afford the yearly property tax. Refusing a valid would-be purchaser invokes serious penalties...use imagination here. Or Something like this. Which was derived from the sci-fi novel Leo Frankowski's A Boy And His Tank. http://www.baenebooks.com/chapters/0671578502/0671578502.htm