I’ve discussed this before, so I’ll just quote myself:

The Westerlings selling their land is a highly unusual event in Westeros – the only other times we hear about selling land is in the context of the Tarbecks forcing people to sell their land through threat of armed force, so voluntary (to the extent that the necessities of poverty qualify as voluntary) land sales are a sign that the feudal order is in crisis.

It suggests that the Westerlings were falling into genteel poverty, such that their rental income had fallen massively behind their ability to service their debt, and that they were having to surrender the collateral they had put up to secure the loan.

Legally, this could be quite tricky. 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 in 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.

The TL,DR is this: normally you cannot buy and sell land freely in Westeros. The Westerlings selling land and the Tarbecks buying land suggests the feudal order breaking down somewhat in the Westerlands.