Having bought and sold via eBay since 1998, there has been a huge change in marketplace. I believe this change is in the consumer more than the marketplace as I've bought and sold trains here for about 6 - 7 years and have seen the same changes.
1) Most people know nothing about trains. When kids or grand kids are selling off a collection, or a wife, or just the guy that bought it at estate sale, they know nothing other than what it looks like, this means they often list it as brand new (if it is in the box and looks reasonably new in their view). They aren't trying to screw you, they just don't know better. Take this into consideration and look at the photos, ask questions, the seller wants to sell and not have a hassle down the road.
2) When selling anything used at all, expect the buyer to be entirely unreasonable. This comes from their expectation that although you've disclosed everything you know, and you're clearly stating the item is used, they will expect it to be brand new and fully serviced by you forever and you'll be willing to give them a full refund forever too. This expectation has come from a retail world of "no questions asked" returns, the consumer expects it, even on used trains.
3) Some folks just can't help themselves, they buy something change their mind and want to return it. These same people do this at retail stores every day. You can't change the tigers stripes.
4) There are fakes, scams, etc., everywhere these days. Do what you can to limit being on the losing end. Never send money via PayPal friends and family unless you know the person in real life. If they need you to cover the 3% fee, just add to the transaction, but don't send F&F. On this forum someone sold me an item a few years ago, guess what never showed and the fellow dropped off the earth, when I contested the sale with PayPal and American Express they both advised it was a "gift" and therefore no action could be taken on their part.
Most of these items apply in person at a train meet and online, it is not buyer beware or sell take cover, but just some basic expectations which if consider will help keep the stress down. When a sale does go smoothly and I don't hear from the buyer for weeks after about every issue they have (forgot that one you're their private service tech for life) like where they didn't change the switch from DCC to DCS, or know how to add an engine to their remote, or that they have to add smoke fluid, or lube, etc., it is a joy filled surprise. The same is true when buying, when it show up, looks like expected, runs, etc., what a great day.
Thankfully the old 80/20 rule applies, 20% of sales or purchases are 80% of the trouble.
Happy buying or selling --
Jason