When it comes to selling, Honesty is always the best policy. If a pony is flawed, do your best to describe it honestly and ensure your pricing reflects the flaw. If you are unable to ship on a certain day (48 hours after payment is what I generally go by) message the buyer and let them know shipping is gonna be delayed for a day or 2 for what ever reason - most will happily accept a delay due to childcare/work commitments for example, as long as its properly communicated. Even if its a big family emergency like a hospital dash, buyers will understand. Be honest with your postal fees, if I have quoted over £1 higher than it actually costs, i refund the difference. Its not a lot of money but its the thought that counts and will keep buyers coming back.
Good quality pictures are a must. take plenty of them. When I sell, I take photo's from all angles and focusing on flaws. BUT when I post the sale, i only put 1 good one on. It saves the buyer having to flick through a bazillion photo's. If they buyer wants more detail, they can ask and you'll have them ready to hand.
Give the ponies a small groom before the photo-shoot. A light hair brushing and wipe with a damp cloth should be all thats needed. Any more in depth cleaning (talking de-heading and oxy cream etc here) should be left for the buyer. Not only are you not wasting time and effort on a pony you are not keeping but if something should go wrong, you may not be able to sell it for the price it was worth before you started the first place.