Believe it or not you are more likely to sell a horse that is slightly too expensive than slightly too cheap. Put a higher price tag and let people haggle to your original thoughts
Have you bought/sold many horses already? Do you know what you're buying when you buy? If not, then the very first thing you need to do is learn. Until you are able to find a diamond in the rough, you may find yourself losing more money than gaining. Flashy horses are not always good horses.
Learn how to assess horses, both ridden and on the ground. Learn when something doesn't feel right, or a horse is mildly lame. Learn to recognise the signs of a drugged animal. Learn to recognise an animal's strengths. Figure out which breeds tend to be better for which activity, but still treat each horse as an individual.
The most important thing when selling horses these days is being honest and trustworthy. There are too many ways the world will find out if you're dishonest, and while there are still a great deal of dodgy horse dealers, they are getting away with it less and less, with more court cases being won. Via the internet, buyers have more ways of finding out who is good and who is bad. There are some well known ones in certain countries, and it would be worth your while finding out who they are in order to avoid them. You don't want your name associated with them.
Be prepared to make a few bad investments. Sometimes it just doesn't work out. Sometimes, through no fault of its own, a horse just takes a while to sell on again.
Learn to analyse riders and potential owners. If you sell the wrong horse to the wrong person, it's going to come back on you. You may not have really done anything wrong, it could be a good horse. People tend to vastly over-exaggerate their experience though, and refuse to admit they've overhorsed themselves. It makes life far easier if you are able to tell someone whether or not a horse would suit them. A horse could be as quiet as a lamb when rode 5 days a week, but the new owner might only be able to get to the yard 1 day a week. A rider might tell you that they've jumped 1.10 before, but they might not tell you that it's only ever been on experienced school horses that they only have to sit there for. Assume everyone is a novice unless proven otherwise.
Trials are a nice idea in theory. In reality, they can cause a world of trouble. I've known people (not necessarily just dealers) who have gotten their horse back at the end of the trial and it is lame, or starved, or has been in some way compromised. For you, that's time and money lost. If offering a trial, offer it on the yard.
Be prepared to take a horse back within a reasonable time frame. Sometimes it takes a few weeks for an owner to realise they don't click with their horse, or maybe there is something wrong with it. That's okay and taking the horse back is a gesture of good faith that is appreciated all over (although, if there's something wrong with the horse, even if you didn't know about it, you have to take it back as it's considered a commercial sale).
Watch out for chancers. If someone comes back to you in 6 months time and says that the horse is suddenly bucking, there's a good chance it has nothing to do with you.
Read up on the law of your country. It can stop you from doing something wrong, and save you if you're accused of doing something wrong.
Check passports! It will save you a lot of time and money faffing around getting the horse re-passported again if you get the right one sorted in the first place. And if you end up with a high volume of horses, find a way to organise them.
Befriend your local equine professionals. You'll need them. A lot. It's best to start off on the right foot with them. Unfortunately, that also means the local knackery. It's a tragic fact that if you have livestock, you will have deadstock, despite your best efforts to keep them alive. This may be from buying a horse you didn't know was sick, or accidents that happen on premises that can't be saved. Horses are particularly good at getting themselves into bother.
Have a plan for disease outbreak. Having horses in and out of the yard on a regular basis means a higher risk of disease. What can you do to prevent this (isolation stables, etc)? What is your contingency plan if disease does break out?
I think your best bet would be to research credible and trustworthy dealers and work for them for a while, if you haven't already.
Usually when I’m looking to buy a horse, I will go look at it, look at basics like form or any old injuries. The age can also have an effect...
Then, I will never take a horse with out at least a 3 week trial to jump it and see what it can do... then 80% done I’ll get my vet to come check it out and maybe a x-ray...
Hopefully this was help full