I've had surprisingly good luck buying used train stuff for my layout--even track and turnouts. Most sellers are trying to be honest. They don't want the hassle of an unhappy buyer. Bad buys?--sure I've had my share. But the back story of buying used is that "if you're not prepared to work on it, then you shouldn't buy it."
I came to this hobby from another one--antique radio. There ain't nobody going to work on your old radio (well, there are a few but that is a whole 'nother story). So you MUST work on it yourself. So when I had to work on my used train "buys", it seemed normal to me. But I guess others have different expectations.
Here's hoping all your used locos work great straight from the hands of the mailman!
Don Merz