Selling an instrument is partly governed by size, and demand. You'll have no trouble shifting a flute or recorder - there's always a healthy market among junior and high schools who'se students are looking to learn. A genuine lifesize alpen horn may present rather more of a problem. Pianos are best off-loaded to a local dealer - check your evening paper and Yellow Pages. Failing that, newsagents' windows, supermarket notice-boards, the Music Department of your local college or university, and don't forget to ask your local music library - they are in touch with musicians' groups and may be able to find you a buyer. Usng one or all of the previous methods, you should be able to shift just about anything - but make sure you get an idea of market value first, to avoid either under or over-selling your instrument(s).