Can you ask the seller for more details on the tree width? Most people who sell on eBay
really have no idea what their tree size is. Most will look at the gullet width and think it's the tree width.
It would be a shame to let such a lovely saddle go (if you really like it), without really knowing the tree size.
Can you get seller to give you a gullet measurement from D-ring to D-ring? Then ask if they can lift the flaps to find the tree points, and ask if they can estimate the angle between the tree points, or send you a picture straight from the front with the flaps lifted so you can try to eyeball it yourself?
Have you taken a wither-tracing of your horse so you know about what shape/size tree and gullet you need?