I purchased most of my trains and accessories used. I've made purchases from folks here on the OGR Forum and have had fine results.
My E bay purchases ( since 2007 ) have rarely disappointed me and the couple of times they were disappointing, the seller was always good about taking the item back. Usually this happens when the seller innocently listed the item as "O scale" when in fact it was O gauge... usually from a person unfamiliar with O gauge vs. O scale trains ... such as someone selling their relative's collection upon the passing of the relative. I had an experience with a seller selling some " brand new" Weaver freight cars as a lot of three. Once the cars arrived to me, I discovered that the cars were indeed "brand new" still in factory sealed boxes, however, there were no trucks or couplers on any of the cars. The seller, was someone not familiar with model trains, was extremely apologetic to the point of embarrassment, and agreed to take the three cars back. The seller told me that she had acquired them from an estate and never dreamed they had no wheels or couplers. My personal lesson there; to always view the entire car not just thru the see through Weaver boxes they are packed in, for those see thru boxes do not expose the wheels or couplers. The thing is, I was not upset about this transaction, but the poor seller was upset and embarrassed to no end. I assured her that she was a fine honest person and not being a train person, she rightfully assumed the cars were brand new and fully intact. A totally innocent mistake on her part.
Of course I've bought lots of my rolling stock at train shows ( remember those? ... they WILL return! ) and on E bay. I tend to like " Best Offer" on the Bay. I've made some ridiculously low offers and have won the deal. I've done this at train shows too and the buyer has accepted my offer. This has happened to me several times. I can't say that I've ever been burnt by a seller either on the Bay, at train shows, or any other venue.
I do stay away from on line sellers that state " looks brand new but has not been tested". Once I see those words I move on.
At train shows, I always test locomotives, before handing over the cash. Thats just good common sense. If the seller does not have a test track at their booth, then I have them walk with me to one of the show's test tracks. I did have a seller who told me that a certain MTH PS 2 locomotive, which I was very interested in, worked fine. Once we began to walk over to the show's test track he started mumbling that the transformer the show used was problematic.... and that he knew that he should have brought his own test track and transformer. We arrived at the test track, turned on the juice, the locomotive would not move..... even after he tinkered with it. Again he blamed it on the type of transformer the show was using ( don't remember what kind now but it was a standard digital transformer ... either Lionel or MTH ) and told me that the locomotive worked just fine before he left his house ... which was 300 miles from where I live. He was giving me the hard sell ... telling me that if I took it home it would more than likely work with my Z4000 ... and if not I could return it to him. I told him to bring his own test equipment to the next show along with that particular loco and once I saw it working I would "think" about buying it then ... if in fact I had not found the same loco somewhere else. Since then, the same seller show up to those shows regularly, but with no test track. He may be an honest guy, however, because of that one experience I'm not comfortable with buying trains from him. I choose to keep on walking by his table.