There are all kinds of rescues, and I’d find out something about how the rescue operates, as well as what their contract is like. The rescues that don’t do the “saved from kill buyer auction”, but instead do mostly law enforcement seizures for abuse and/or neglect (most are for neglect), can end up with horses of any kind including some quite nice horses. A lot of time the neglect (read: starvation) problems comes from a change in the owner’s circumstances that the owner didn’t face up to, such as job loss, old age, death and inheritance by non-horsey kin, hoarding (more common than people realize), etc. And some just irresponsible, uncaring people.
Nice horses can end up in rescue for reasons that have to do with human crazy, rather than horse crazy. And human misfortune, with the horse as an innocent victim.
A lot of problem horses in rescue are very fixable with work and time. Most of the problems are due to owners who didn’t know what they were doing, not lunatic horses (which are rare).
Because most rescues have a buy-back clause if the horse doesn’t work out, the rescue is motivated to make a good match with an adopter. That doesn’t mean things always work out, but they do try. Also, the adopter gets the recent vet, training, feed, etc. records, which isn’t always the case with a Craigslist horse.
Some rescue contracts do allow an owner to “sell” the horse on, so long as the next buyer also signs the rescue contract. In these cases the rescue intends to be a soft landing if things go wrong for a horse once again. The rescue doesn’t really want the horse to come back on their dime, unless the horse basically needs another rescue.
The contract may allow the adopter to set a selling price and keep the proceeds, so long as the next owner also signs the rescue contract and is agreeable to its terms. Which may be simply cooperating with un-intrusive inspections for a couple of years, and sending photos & news back to the rescue once a year.
Rescues should inspect new adopters at least twice a year for the first two or three years, to be sure the horse is doing well and everything is working out. Some of the rescues don’t continue inspecting after that, but they do want yearly updates on the horse’s welfare, and to know if anything adverse occurs or if it dies. If you think about it, it wouldn’t be much of a rescue if they didn’t keep up with the horses they rescued. 
The point of a rescue is to be a safe landing for a horse, not just a transition point back into an unknown future. But the best-run rescues don’t want to alienate potential adopters. They know that statistically horses do go through a series of owners in their lifetime. And they need horses to go to well-qualified, caring adopters, so they can rescue more horses. 
