Good point on obtaining items. Local shows are nice but ebay offers the entire country/world as a 'shopping flea market'.
I have lionel/MTH/3rd rail, etc but I'm mostly into HO modeling and I can purchase very nice (most have never been run) brass locomotives. The price of most brass engines are around what they were selling for in the 70's-90's so, with inflation, they are extremely cheap. Usually, at flea markets, anyone selling brass engines usually has them way over priced. The same with a lot of Lionel products: a lot of stuff that was thought to be hard to get, isn't.
Without ebay, I would never have been able to get my collection of 'train stuff'.
Some items are very hard to find. I have two geiger turntables that were made back in the 70's. On ebay, someone had a complete motor mechanism for 10 bucks that I now have as 'backup' for spare parts.
One can input 'saved searches' of items that you are interested in and ebay will notify you, via email, when one is for sale. Sure saves me a lot of time, checking every day/week to see if that item is available.
One can also click on the 'sold' option to see what 'like' products sold for to see what current prices are for an item. Although the price can vary by time of year and even by week. I've bought items that I thought was a very good buy, only to see, a few weeks later, people paying a lot more for the same item. This is in relation to brass locomotives where the overall quality is a lot narrower 'field' versus 3 rail items that can vary widely on condition.
My view on buying, I pick, ahead of time, what I think the item is worth to me. If I get it, great, if not, there will be more of the same, sold later. Items that I really want, and don't come up often, I will pay more for, but that is rare.
I also find auction houses a very good source for items. Stout seems to be the best for knowledge of 'train stuff'. When viewing products, it's obvious some 'houses' have no knowledge about trains. Stout puts in lots of pictures and a well detailed description of the item. One must remember to add the 19-25% that they add to the price of the item plus shipping. One can put in a bid ahead of time, which is nice, as you don't need to be 'online' to win and, again, if I win, great, if not, keep looking.