Hey Tony,
I only want to share my experience with eBay with you. I have had great luck with eBay.
With limited production in new O gauge eBay was a must for me to build my inventory of rolling stock and a few locomotives on eBay.
The buyer protection is as LASM said the best in the business. I have bought over 200 items on eBay and I have only needed eBay's assistance with 2 purchases. Both worked out in my favor.
When I first started I did not do as well as I do now but that was purely my own fault for not knowing market values of what I was buying. Now I consistently buy Lionel products at 75% off of book value per Greenberg's Lionel Pocket Price Guide. if the price goes beyond that I seldom keep bidding, but I occasionally will go higher if it is something hard to find.
I have bought several HO items and about a dozen Tyco HO locomotives. I was able to repurchase the first train set I ever had on eBay. I lost the original after my divorce. That train is not worth much in real dollars but it is priceless to me. After a bit of work it runs like new.
I am willing to buy used Lionel locomotives on eBay because most parts are fairly easy to come by, but I shy away from used HO locomotives now because I don't need any more projects.
I recently had a limited edition 1974 Lionel GP20 that the power truck locked up on. I searched for parts and found it would cost me at least $120 to repair or more to replace the power truck. I turned to eBay hoping to find any Lionel GP from the era to cannibalize for parts to make repairs. I not only found one, I found the exact Chessie B&O GP20 locomotive I was looking for. It was of course used and the seller was not even sure if it worked, but I took a chance and paid $70 including shipping. To my surprise it looked brand new and did not even have any marks on the wheels from ever being run. Not only did it run, it ran perfectly. I did not have to do anything to it other then to oil and lube it. It came in the wrong box, but I had an original box from my broken locomotive.
I found a 1974 Lionel Bicentennial U36B Seaboard Coast Line locomotive with all 13 spirit of 76 boxcars and caboose in nearly mint condition. Fairly hard to come by complete with all original boxes and in such great condition. I paid $225 including shipping. The complete set lists in the book for over $1100. I believe I got it so cheap because I was the only bidder and the seller had less then 10 ratings. I believe people either did not know of the listing or they were afraid of doing business with a new seller. I knew eBay would stand behind my purchase and it worked out great for me. I run that train for hours and hours every 4th of July.
I could tell you about a dozen other stories of things that worked out even better then I expected.
For me the key to eBay is to carefully examine the listings and set my price and not go over what I am willing to pay. I wait to leave feedback for after I have received the item. If I find the item is not as it was described I contact the seller and 9 times out of 10 they are willing to make a concession to keep their good ratings. If the seller won't be fair I contact eBay and open a claim. I have only had to that twice.
Thanks to eBay I have been able to buy some of my most favorite items.
I would never tell you to do anything but I wanted to share my experience with you. eBay has been a great value to me.