I've had a lot of good experiences buying saddles off eBay. Just be sure to ask the seller questions before you place your bid.
Tree: Any seller should guarantee that the tree is sound. Ask them to. If they won't guarantee it, then don't bid on it. When you receive the saddle, check it (or have your local saddler/tack shop check it), if it is found broken, then the seller should accept it for a return including all shipping charges (get them to agree to this in their guarantee before you bid).
Leather Condition: Ask them specifically if the leather is stiff or dry-rotted. If they say something like, "it could use a little more breaking in," then it's probably stiff from sitting around for a while. If the saddle is used, it's probably already broken in and the leather should not be stiff. Better yet, ask them for a photo with the flap rolled up.
Also ask if it has any loose or broken stitches. If it has safety stirrup bars, ask if they bend freely, but also stay firmly in place. Ask to see a photo of the seat (top-down) to check for any seam separation or excessive wear along the twist where the seat meets the skirts.