Make sure it fits! Ask questions.

Many people buy saddles, both english and western strictly on the brand name, amount of silver or other popular considerations. However, no saddle regardless of cost, name or attributes will please you if it galls your horse or yourself.
You need to ask questions of the seller before you bid. Sellers are more than happy to answer your questions. They do not want negative feedback because you assumed the saddle would suit your purposes.
Be sure and mention what breed of horse you are outfittting. This helps the seller to determine if the saddle is in the right ball park. If you want to see if a particular saddle will fit your horse, take a stiff piece of wire and bend it accurately to the contours of your horses wither/shoulder area. Make note of the distance from the center of the bend (top of withers) to the ends (the ends should be exactly the same distance from the bend). Make a second measurement of the distance from one end to the other. In other words you will be forming a written "triangle". Give these measurements to the seller and they will determine whether the saddle is in the right ball park. Your horse will thank you.
It is also wise to let a seller know if you are outfitting an Arabian or other shorter backed horse. I've seen huge western saddles on Arabians and the rear skirts actually rest on the hip bones.
Also, if you have access to a similar saddle, try it on your horse. Use a thin pad/blanket and saddle the horse as for riding. Work the horse on the lunge until it develops a slight sweat. Remove the saddle and look for "dry" spots on a wet back. These dry spots are "pressure points" and indicate that the saddle does NOT fit the horse. If the sweaty back is even and consistent, then it indicates that the saddle fits the horse well. Also be aware that the gullet must clear the horse's withers with the rider's weight. It should be no more than 2" above the withers with the rider mounted. Any more than that indicates that the saddle is too narrow and will cause problems on the withers and upper shoulders. Use caution if buying a vintage saddle; many of them were made with a higher gullet than today's saddles.
Once you've determined the saddle in question will fit the horse, be sure that the seat size will fit you. In today's world there are saddles for every discipline and every section of each discipline. If you are looking for an equitation saddle, then a cutting model might not work.
The main point here, is to ask questions and ask for help. Your seller wants you to purchase and be a happy camper.