The first thing you need to do is decide what level horn you want-- student or pro. New student horns are fine for beginning players, but beware of buying used student horns; not that they're junk, but because they often have not been properly cleaned or cared for. You remember how it was in high school, the things kids did with their instruments? And I don't mean "and this one time, at band camp..." A seller may state, "Good condition, used only in high school, stored in closet ever since." But what condition was it in when they put it into the closet? Chances are it's been sitting there for how many years corroding on the inside because they played the horn after eating and never had it professionally cleaned. Kids just don't know enough to do that. Wind iinstruments corrode on the inside from the acids in our mouths the same way teeth decay from going to sleep without brushing our teeth. I've had people GIVE me their old student horns which they thought were in good shape but couldn't sell, and the slides were so gummed up that they were practically rusted solid. If you're looking at a used horn, whether student or pro, make sure it's been (or will be) professionally cleaned. Slides can be tricky and often need service, too. A knowledgable and responsible seller will do this anyway-- professional musicians know enough to have it done. If it's a pawnshop, you'll probably have to ask them. Don't trust them just because they say, "Great working condition." They don't know enough to be able to tell. That's like a car salesman telling you a used car runs great because it's clean and it starts. Of course, the best thing is to try the instrument out personally if you're close enough, but if not, get a guarantee so that you can get a refund if it turns out to be in bad shape. You can bypass this problem by buying a new horn, but make sure it's a reputable brand such as Conn, King, Bach, Edwards, or Getzen. Stay clear of the ones from China that have ads like: "Wonderful beautiful rare trombone." Also beware of the no-name ads that say the horn is "like a Bach 42 (or Conn 88H) but at a fraction of the price." They make LOOK similar, but the quality isn't there. If you're at all serious about playing, a good horn is worth the investment. About price: you should expect to pay well for a good horn, but don't fall into the trap of becoming obsessed with having THAT particular horn and getting caught up in a bidding war. Once you've decided which model(s) you're interested in, do a Google search for those models to get a good idea of what a ballpark price is, given its age, condition, features, etc., set a price limit that you're comfortable with, and ONLY bid up to that point. You may not get the horn you want the first time around, but good horns are newly listed EVERY DAY, so be patient. You may not even find the lowest prices on Ebay (hope they don't disqualify me for saying that), but it's true-- some people way overbid on "designer" brands like Bach when they could find a better price on another classified website where people aren't bidding against each other. Doing research before bidding is the best way to get a quality instrument without overpaying. And if you end up with a horn that isn't right for you, or the kid doesn't stick with it, you can always turn around and sell it.