My fear with being able to sell a leased horse would be the possibility of someone trying to earn double money off of a horse - lease it out for a year for $X, then turn around and sell the horse for $Y.
As pointed out, for me, this is less of an issue with broodmares as with racing stock. Since racing stock has limited times for peak usage (racing), and it takes a while to figure some ponies out, I would be irritated if I bought a horse and could not run it, especially if the leaser were to go inactive/stop running that horse.
However, if there was a way to make the lease information more prominent - even something as simple as making it bold or red font on the horse’s page when the horse is up for sale, I think that would help my peace of mind. After all, if you knowingly read that the horse is on lease until X day, you can make an informed decision (. Sure, in a perfect world, that information is there and I would always take plenty of time to notice it. Unfortunately, my powers of observation are not always acute. There are scenarios where purchasing a racehorse that was out on lease would not bother me (right price, lease is about to expire, horse is SUPERAWESOMEAMAZING, I estimate that the horse isn’t near it’s peak age anyway based on siblings, etc.)
ETA: A better way to notify the buyer of the lease? On the page where it confirms that you want to “buy the horse for $X” and makes you confirm, you could have a simple “*This horse is leased to Jane Doe Farms until MM/DD/YY”.
As a leaser, there are no scenarios where I would be bothered if the horse were sold, as it does not affect the lease. I don’t expect anyone to sell me a horse just because I leased it from them, though that scenario is always nice (especially if the horse runs well). Though I might be a little offended to not get notified of the option to buy (assuming I had kept the horse active), I know I could always try to start negotiations with the new owner…
Also, instead of the option to cancel the lease if the leaser goes inactive for X days, maybe it could be a scenario based on # of days without running in a race, maybe 60 days or so. That count would of course have to be reset after injuries. I’ve just seen horses go to active homes, then just sit idle, despite regular log ins. I’m sure that’s frustrating to the owner…