I bought my first saddle off of the internet this winter (2006). It is a used, older Broken Horn saddle. I wrote out a checklist of all the things I needed to know about the saddle, as well as the things I would be inspecting the second it got into my hands so I would not miss anything.

• Check whether skirts are even front to rear
  check all hardware especially screws and rigging
  Check wool lining
  Check all stitching
  Check for lumps under skirts
  Check for cracks and wear in the leather
  Look for cuts and discolorations in the leather
  Ask about its care, and determine whether it was oiled and what oil was used if possible
  Check all straps
  Check Dees and Stirrups to see whether they are cracked
  Check silver for looseness and wear
  Check rig catchers

I bought the saddle from TackTrader.com and like eBay you really have no guarantee that your purchase will go smooth. So proceed cautionsly. I first sent an email asking some questions, the answers came back promptly and were detailed. I then sent an offer (right away), with my phone numbers for the owner to contact me for more information. eBay is a bit different in that you might not get to phone the person, I was prepared to ask for lots of detailed photos if my find was from eBay.

The owner called me at work the next morning. She had another offer but mine was highest, so I had first dibs. We chatted and I went through my "list" (below) with her. I also spoke with her at length about her horses and showing, to get a feel of whether she was a trustworthy soul. After a nice long chat, I decided to go through with the deal. The owner was not internet savvy so I was stuck with sending a check. I made a mistake and sent a cashier's check via regular snail mail, then fretted for 5 days till she got it. In the future I will do the following when making an online private purchase:

• Push for Paypal (with eBay this will not likely be a problem)
  If not Paypal, then send a PERSONAL check (so I can verify funds)
  Send the personal check CERTIFIED mail  

More tips on used saddles to consider:

• Don't fall for the cheap saddles, stay with name brands. My aunt learned to ride last year and she got all excited about a "Deal" so she bought it without asking me. It was a couple hundred dollar package deal, she had no idea at that time that saddles were so different. She had been riding in my Bob's show saddle for about a year. She knew immediately when she opened the box and peeked inside the horrible mistake she had made. Luckily she had made sure it came with a 30 day money back guarantee.
• They last a long, long time, I bought a 1992 model and here it is being used by me this summer as a show saddle
• If you are careful with your research you will likely be able to find an older used saddle that still has the "look" of the new saddles, note my saddle photo, it has the big skirt and rounded back like the current reining saddles have today.
• It WILL have flaws, just know what they are and be sure you can live with them. For Example, this used saddle I bought had damaged leather on the right pommel. This was insignificant to me
• There are many parts of the saddle that are easily replacable, namely, the girth, latigo, billet and stirrups. I didnt like the bell leather stirrups that came with my used saddle, but I found a SCREAMING deal on some engraved aluminum ones from eBay. I bought them to put on the show saddle, then put the old plain ones on my new - used saddle

Good luck with your purchase!