I just dealt with a few shotty people when looking for a pony last summer. I went out to look at a 'welsh pony yearling' and it stood out as what not to do when selling.
Please state WHICH registry the horse in question is registered for. If ad says Welsh pony, and registered, most people would assume they'd be registered Welsh and not with some obscure pinto org as a "pinto color carrier" when it was a solid bay.
Disclose medical issues that will require surgery (in this case hernia), including a price that reflects that. I can see where seller may not want to advertise that, but I like honesty, and its just rude to NOT disclose that before someone comes to take a look, you can say something like "vet says it will be an easy fix and is a common surgery, just don't have the money for it right now" or something... Anything would be better than arriving to look at a pony that has a very obvious 2" diameter hernia...
Age, height, breed, location. I would like at least one confo shot from the side, reasonably squared up (no feet chopped off in pic, that just makes it seems you may be hiding lack of farrier care?) and if broke to ride or drive, a working picture of it is nice. I don't need a video, if I am interested beyond whats been covered, I want to go see the horse in person, and can assess that then, but of course they are nice.
If you have an ad with no pictures I will ignore it unless there are specific bloodlines I like. I can ignore small spelling errors, but if it seems like a 10 year old wrote the ad I will pass it up.
And I will agree with taking pictures in front of something that is at least not distracting. I would rather see a picture in front of a house or garage than one in the pasture that's next to what looks like a scrap yard.