Horse seller here. I have been the main or ancillary seller in 10 sales this year and if I had access to more horses, I could have sold many more. Of course I deal in a specialty breed in low supply and high demand. However, the horse market for a safe, pleasant horse is pretty good regardless of the breed.
J-Lu gave good advice. You need good photos – horse standing square conformation shots, You need a video of the horse being mounted and then at the W/T/C. If the horse does well on trails, so trail video is icing on the cake. The video does not have to long but show the gaits and the horse’s way of going. Height, age, training, and location. Buyers are coming from all over not just locally to buy horses.
While FB does ban ads for animal sales if you word your ad so that it says something like For Your Consideration, does not say SALE anywhere and instructs to PM for more info, it will be accepted. I also suggest using a paid service like Dreamhorse. Pay the $25 for the photo. Then get ready to work because you will get queries and you need to stay on top of them. The will be a scammer or two, but most will be legitimate. They will contact by email, by text and sometimes by phone. Just be prepared to be a parrot because many buyers have readying comprehension issues and will ask the same info that is in your ad. I have sold horses to buyers sight unseen, but more often than not, they will come try the horse. I recommend it personally. If the horse seems like a good fit and the buyer is interested, get references. CHECK references. Also, during the time the buyer is there, you should have gathered information on their background, horsekeeping set-up, intended use, etc.
One point that I want to make as a sales agent. I never, ever, ever sell a horse without the owner’s permission. I gather all information, check references and relay the offer to the owner. I give my opinion about suitability. The owner chooses, not me. I have said no to buyers on behalf of the owner. So using an agent is not giving up control over the horse. Your rights as the owner should be stated in the consignment contract. Of course, if no contract is provided, then you might want to think twice about using that agent.
If you have the time and energy to sell the horse yourself, then go for it. Absolutely that will be best for you.