The Craigslist posting has been removed.  If you're looking at Craigslist horns, make sure you are able to see it in person and ascertain its condition before buying.  Craigslist has essentially no buyer protection.  eBay is a good place to start a used horn search.  The more you know of closing eBay prices for comparable horns, the better idea you will have whether an asking price is reasonable or not.  eBay also has some protection for the buyer if you decide to go with an online sale.

Probably the safest route to go for a used marching band horn would be a Yamaha YAS-23, or maybe a Jupiter.  The Jupiters tend to be lower priced, but you have to be more careful buying them.  Their early horns developed a poor reputation, but their more recent horns (since about 2000) are solid.