You can see her spine. Being 20 is no excuse for that, I know horses in their 30s that look 10x healthier. Not just weight, but general health. If it is indeed tooth problems or parasites, then they should be getting it fixed as best as possible, not just leaving it.
I don't know what a rescue can do in this case. If she wasn't for sale, then yes, I'd call a rescue. But because she's for sale, she's potentially going to a new home, and hopefully a better one.
If anything, maybe contact a rescue and just ask for some advice, maybe see if they know of any experienced horsey folk looking for a rehab project.
Satin Reign aka "Misty"... my life, my love, my everything.