how do I sell my horse to someone in the sim 3 community? like how do I send the horse to them?
You have to save your horse in your game as either .sim or sims3pack, mostly prefer as a .sim. Then you just send the file to the new owner's email.
Another tip: When you are selling a horse, please, please, make sure you know what markings you used on the horse and can provide a list of them to the new owner so the horse will appear properly in their game too. I've bought several horses lately and I've not been able to even register some of them because the owners can't identify what custom markings they used. Some sellers are good, and can either tell you what they used straight away or will help identify the markings, but some people ... aren't.