The first hard rules of Saddle Fitting are the correct curvature of the saddle tree and the panel configurations matching the outline of the horse's current back condition. Each horse is unique and should be treated as a single entity.
In consideration that the head plate and tree itself is sound, other factors must be taken into account prior to using and investing in a Pre-owned saddle.
Over time, the Pre-owned saddle will take the shape of it's mount and subsequent asymmetries. You must have an understanding of the correct balance and length of tree points; use of the pommel to mount, may lead to the tree points being torqued thus setting them unleveled and unbalanced. You may be placing a "crooked" tree on your horse. Without proper recording of prior tree work, you will be unaware if the tree has been widened or narrowed in the past. The head of the tree should only be adjusted one width +/- for the life of the tree. Use of inferior adjustment machines by novice hands may set the tree points at unleveled angles. Head adjustments are a very serious modification and should be done by a Qualified Saddler only.
The symmetry and balance of the stirrup bars must be examined. Any unevenness will cause Rider Influence to become skewed. A crooked Rider may create a crooked Horse. If the bars are uneven, the original placement must be evaluated, as well as resetting to plum.
We have learned that design flaws from older practices actually damage the horses back and impede muscle recriutment. A glaring example is the width placement of the panel whereas insufficient room within the gullet channel and at the back of cantle pinches at the spine and increases pressure onto the ligament and muscle structures flowing from dock to poll. There should be clearance on the top and sides of the spinous processes. Ideally, one would look for a minimum of two inch spread between the panels. Too wide of a channel will give insufficient bearing surface provided for the horse.
Saddles are man-made products, using leather and thread. They do have a shelf life.
The prior treatment and conditioning of the leather may have broken down the integrity of the leather. Panels that are cracked or torn are highly unsuitable. If flocked, understand that the tensile strength of the fibers may be deteriorated. The materials used for flocking must be evaluated. Older saddles may have carpet fibers, or poorly constructed synthetic medium which will actually hold heat and ball creating triggers points affecting the bearing surface area of the horse's back. If foam is used, that too will degrade with time, elements and use of conditioners that become absorbed into the medium, adding weight and instabilty.
Within the last decades, Saddle Makers are taking into consideration of the advances from Medical Technology and Biomechanics. By applying these variables, British Saddle Makers, and other renown Saddle Makers now craft an Anatomically Correct Saddle.
The British Standard (BS6635) of design and calibrations is one of the many examples of the Accountability the Society of Master Saddlers brings to the Trade. In new saddles, from the manufacturing of the tree, to number of rivets used, to the symmetry of the finished product is under scrutiny. Revised in 2015 to upgrade the quality of materials used and workmanship governed , the British Standard (BS6635/2015) creates a well balanced, level , finely crafted Saddle.
Please do not be swayed by "Price" or "Convenience". Allow your choices for correctly fitting equipment be dictated by knowledge, skill and expertise.