If you're looking to buy cheap, then beware. You could get a bargain but you could end up with a lot of work, if you don't know what you're doing.
I was going to say that there?s no easy answer ? but on reflection there is ?and it?s this. If you don?t know what you?re doing then play safe. A bad first experience could put you off for a good while and that would mean missing out on a whole lot of fun.
J2, there's a lot of good advice been given here - pristine cabinet queens v. oil and crud coated bangers, with scratches, scuffs and scorch marks will tell you a lot before it evens steams.
It goes without saying that the older the loco the greater the potential for problems so check the wheels - if the plating's wearing away or is totally worn off it shows it's a high mileage runner, which means everything else has had a good run for its money, so cylinders, jets and burners would need overhauling.
In addition a boiler and safety valve test would also be at the top of my list.
Inexperienced modellers should only be buying second-hand from a reputable dealer, and one who provides a warranty.
However, if you like a challenge, relish the excitement of a gamble and want to learn from your mistakes, this is one great way to do it. Just be prepared to spend a few years getting whatever you buy finally to run right.
And finally, a sage, not-quite-so-young-anymore, man known to us all (and having a faint whiff of the sea about him) summed up buying live steam off eBay as being the place where someone sells a steam loco that they know is a pup and don't want any come-back....... however it can also be the place where a legitimate model is sold.
As always, caveat emptor.