I usually have horse clean and ready waiting in crossties. Legs bare for buyer’s inspection. Allow buyer to watch or assist in tacking, as desired. Demonstrate that horse stands quietly, doesn’t eat your arm off when girthing, etc.
Make sure anyone intending to ride signs a release.
I make it very clear that buyer is under NO obligation to ride or continue if they decide the horse isn’t right. No pressure to canter, jump, or keep going if it isn’t what they’re looking for. Some amateur riders feel like you’ve cleaned the horse up and made a time specially for them, and it’s rude if they don’t go through the motions and ride the horse w/t/c/jump before saying No. If they feel at all uncomfortable, or just “not clicking,” I am happy to wish them the best on their horse search and move on. I tell people, if they get on and walk 5 steps, and it just isn’t the right horse, it is not a waste of my time to get off and hand me the reins.
I’d much rather put the horse away, than watch someone struggle, a trainer drilling my horse unnecessarily, jump the crap out of it, and get off saying “He won’t do.” If you know that after one trot circle, that’s okay with me! Yes, I spent an hour bathing and scrubbing whites, pulling mane and brushing tail, and cleaned all my tack with the expensive conditioner, but I, too, have other things to do with my Saturday if the horse is not a good match for you. 
I ride first, show that horse performs as advertised. I might talk about the horse’s strengths and weaknesses as I go, pointing out how I manage the crookedness or minor issue we’ve been working on. I did not use to do this (I wanted horse to seem perfect!) but I had several buyers get on my green horses and say, “OMG you made this horse look so easy! But he isn’t! Help!” I’m great at making a horse look easy with small, unnoticeable aids…so while I ride I will point out when I’m using my inside leg, my outside rein, my core, etc if I think it will help.
Then trainer or buyer gets on, as they prefer. I am happy to answer questions, give tips and assist if needed to help buyer and horse get along, but generally I stay “in the background” and let buyer/trainer figure it out on their own (if they take the horse home, they’ll have to do it on their own anyway). I’m okay if the trainer wants to push the envelope a little to see how the horse responds, but I’m not going to allow anyone to risk the horse’s well-being for a “might-buy” ride.