When did kings start allowing land to be bought and sold?

It’s a bit complicated. To quote myself:

In Medieval England, for example, the feudal principle of “Nulle terre sans seigneur” (no land without a lord) meant that selling land outright, known as “alienation of lands by will,” was actually legally impossible until the late 12th century. (The Magna Carta, for example, says that “No free man shall henceforth give or sell so much of his land as that out of the residue he may not sufficiently do to the lord of the fee the service which pertains to that fee.”) Selling land was legalized by the Statute of Quia Emptores in 1290, although the buyer was “required to assume all tax and feudal obligations of the original tenant,” so the land remained under the same lord as before. It wasn’t until the Tenures Abolition Act of 1660 that those feudal obligations were eliminated.

I think there are arguments you could make for either 1290 or 1660 as “when…kings start allowing land to be bought and sold,” although more accurately it was a gradual process, owing as much to decisions about enforcement and legal fights over whether feudal obligations once allowed to lapse could be invoked later on, that spans the two dates.