Yep, been there Sarahver. Which is why there are two quick turnaround resale projects in the barn....who have been here five and seven years, with no ads online and no intention to put up any ads. So I'm doing really well in the money-making department. Oh, and then there's the gelding I took to sell on commission, but ended up paying thrice his value to purchase him. He's out there, too. Uh-huh.
I did flip a pony once, and make a tidy profit, but I was only co-owner him and therein may have lain the difference.
I've sold a lot of horses through work, too. Some of them I worked with over a long period of time, and I got quite attached. Lately I've gotten reports that a few of those aren't working out in their new homes. Honestly, it makes me physically ill to hear that. I hate it. Makes me a nervous wreck, even though a) it's not my fault and b) there's not really anything I can do about it. Gah.
I will say that it's a whole lot easier when you know they're going a good home. If there was someone I really knew and trusted, I'd more than likely sell or give away one or two of mine. So one key, I guess, is screening homes so you feel good about what you're doing. You'll make a lot less money that way, but I think it's worth it.