It's pretty rare for legitimate horse rescues to troll the kill pens - they already are full-up with owner surrenders and referrals from Animal Control. The "organizations" that advertise, "OMG! Somebody needs to save this wonderful horse from slaughter!" is most likely the kill buyer themself, or affiliated with them, trying to get a few hundred dollars more than they paid and save the cost of transporting them to Mexico.
We have some notorious scammers here in WA. Some of them have a record of taking money from several people for the same horse, taking money and shipping them to Mexico anyway, claiming the horse "got injured and had to be euthanized", etc., every scam you can think of.
All that said, my mare was bought out of our local auction's kill pen, but this was in 2010, when the 2008 financial crisis was still foreclosing on people, the price of hay had gone through the roof, and there were way too many good horses people couldn't afford to keep and couldn't sell. Also, my mare was bought by a friend who is a professional trainer, and the only reason I have my mare is because she has some issues that made her unsuitable for my trainer friend's lesson program.
Fortunately, my local auction was recently sold to better people (after the last owner died and went to hell, I'm sure.) They seem to be more legitimate than they used to be - they encourage sellers to put a reserve price on the horse so they don't go for meat price, and they market the horses (and other animals) on their website ahead of time, including pictures, videos, and whatever description the seller wants to post.
To anyone who wants to rescue a horse - do it through a legitimate rescue, that has a 501(c)(3) designation (ie. a real charity, not a scam) and posts their financial records on their website. These rescues have brought the horses back to good health, have provided vet care, good training including re-training from abusive situations, and they know what kind of home their horses would be a good match with.
Adopting a horse from one of these organizations, means that now the rescue has an opening to take in another horse from a neglect or cruelty situation. Plus, you get a good horse who's just right for you, and you know their history, instead of taking a chance on an unknown horse from a lying scammer.