Many of those instances are selling what one is unable to hold. There was, for instance, ~zero chance that France would end up actually retaining the Louisiana

Many of those instances are selling what one is unable to hold. There was, for instance, ~zero chance that France would end up actually retaining the Louisiana Purchase lands in perpetuity, so the only question was whether they'd get something in return (and they quite needed the money at the time), or lose them in an expensive war later.