I don't know. IMO, when a person buys a horse, that means taking responsibility for it now and being prepared to retain that responsibility for the remainder of it's life if that becomes necessary.
Yes, some older horses can be re-homed, if their owner can find the right home for them...one that is willing and able to take over the responsibility of caring for an older equine. However, what if something happens to the horse during the ownership that deems the horse virtually worthless? They get injured or sick, not enough to need to put them down but just enough where they are no longer "useful". Too many people would be so quick to just pawn them off on an unsuspecting buyer or drop them off at the auction house with nary a care where they end up.
I understand that bad things happen and we can never be certain about anything, but to buy a horse that has questionable marketability now with the clear intention of "selling her in a couple of years when I get my confidence back", when her marketability will be even more questionable, is just unfair to the horse.