Buying a musical instrument on ebay can be a worthwhile, practical investment. Since most purchases will be a used instrument, here are some suggestions about preparing it for presentation. Listed by instrument, I hope this will provide a good guide to making someone joyful over getting a something they can be proud of and get many artful hours from. In all situations, once the purchase has been delivered, find a reliable service facility (talk to the local school music teacher for a reference) and have the instrument checked over. If it doesn't play when it's freshly taken out the wrapping, imagine the disappointment. Don't be surprised if you have to lay out a few buck$ to make it totally serviceable. Look carefully at the case. The latches and carrying handle should be secure, the interior lining should hold the instrument in place safely. As the service personnel about replacing the case if it appears faulty. You can make a case look wonderful by removing the instrument and any accessories and running a vacuum cleaner through it.
Here's my (partial) list, included are some brand names that I'm familiar with and that seem to hold up well But, there is no recommendation implied by it, and of course, it's all relative. Buyer beware.

The following suggestions are for the unfamiliar buyer. If you really want to be sure, take the instrument to a professional music instrument repair shop.For a NAPBIRT (National Association of Professional Band Instrument Repair Technicians) member. Click on Public Access for a service centre near you, worldwide.

The suggestions here are basic. They can't possible cover every aspect of preparation/maintenance. The brand name list is incomplete; it contains only a few of the names that come to mind as trustworthy. . For more information talk to the band/orchestra director or a trusted, professional music instrument service shop.

Piccolo: (see flute brand list below) Not for the faint of heart. Take it to a competent music instrument service shop.

Flute: Yamaha, Gemeinhardt, Artley, Armstrong, Selmer, Pearl, Buffet, Emerson, newer Jupiter

Carefully wipe the instrument down with a damp cloth. Reach in the key work with a water-dampened artist's brush to remove dust or use canned air blaster. Note the little pieces of cork or felt that are on some of the keys; avoid knocking these off. If you should, take it to a music store to have them replace the corks or felts. They're important to the operation of the instrument. Clean the tenons (these are the ends of the parts that meet the other parts) with methyl hydrate or rubbing alcohol. Clean the inside by passing the tuning rod (included with most flutes) and a soft cloth with rubbing alcohol on it, through the instrument. If the instrument is silver plated or solid silver, you can restore some of the sheen with a special cloth available from good music stores. Don't immerse the instrument in water. ABSOLUTELY AVOID USING ANY COMMERCIAL LIQUID OR CREAM SILVER POLISH such as is used on table flatware. That stuff can damage the mechanism of a flute. If the flute won't go together easily after you've cleaned the tenons, don't grease or oil them. It has to go to a service shop. Useful accessories: tuning rod, special polishing cloth, replacement case if necessary.
Note about dents: some dents are harmless, but some can affect the playability or assembly of the instrument. If you see any dents, take it to a repair shop for assessment. Many dents can be removed quite successfully. Pads: these are the soft parts under the keys that cover the holes. They should be free of  breaks or tears, but a trained eye is the best to evaluate their condition. Get an e$timate of the co$t.

Oboe: see Piccolo, above. Brand names: Yamaha, Selmer, Bundy, SML, Lorée

Clarinet: Yamaha, Buffet, Selmer, Bundy, Evette, Vito, Leblanc,

Plastic/composite clarinets: Generally, the same as a flute, but avoid using alcohol on it; it can often affect the appearance of the dark plastic. Use water, but don't immerse it. On the tenons (the corked ends or the receptacles for these ends) wipe off excessive cork grease. If any of these cork tenons should fall off, have a musical instrument service shop replace them. Don't attempt to re-glue them. For checking pads: see flutes above. Clean the mouthpiece by soaking it   in concentrated lemon juice (don't let the cork touch the lemon juice) for about 20 minutes. Rinse it off in warm (never hot) water and dry it thoroughly. If it's still ugly, repeat the process. If that doesn't work, the mouthpiece may have to be replaced from a music store. By the way, if the mouthpiece is branded as Vandoren, H & A Selmer, Bay, Ridenour, Brilhart, Runyon, it may be an expensive one, anywhere between $75 - $150. Treat it kindly. Any mouthpiece should be free of breaks, chips. Useful accessories: a pull-through swab (hanky type is best), reed holder, mouthpiece cap. For checking pads: see flutes above.

Wood clarinets: These are special. To be safe, take it to a service person to check for cracks, loose posts. For mouthpieces and useful accessories, see plastic clarinets, above. The pads on wood clarinets often become discoloured, a dark brown. This is natural and will not affect the playability.

Metal clarinets: these are only really useful as decorative items. They make great table lamps.

Saxophone: Yamaha, Keilwerth, Selmer, Bundy, Evette, Buffet, newer Jupiter, Vito

See flute service, above. For mouthpiece, see clarinet above, but ignore the cork comment.Some of the keys have pearls or pearl lookalikes on them. If these are missing, a music instrument repair shop may be able to replace them. Check the cork on the sax neck. It should be solid, not loose, no nicks or pieces missing from it. The mouthpiece should fit on it snugly. If necessary, have this cork replaced at a repair shop, but take the mouthpiece with you so that it can be fitted properly. Useful accessories: pull through or "shove-it" swab, mouthpiece cap, lacquer polishing cloth, neckstrap (look for a comfortable one!), reed holder. Dents: see the note in flutes, above.Pads: see flute service above.

Trumpet: Selmer, Bundy, King, Yamaha, Bach, newer Jupiter, some older Bessons, Holtons, F. E. Olds.

Wipe the instrument down with a soft, damp cloth. Push the valves down. They should return quickly on their own. If you hear a clicking sound, finger-tighten the caps, these are the threaded parts. If the valves don't go down smoothly, return slowly, or sill click, they may need servicing: take it to a music repair facility. The slides should move in and out easily, but with a small effort. If they don't move at all, take the instrument to a service shop. Repairing dents: see Flute section above. Inspect the mouthpiece carefully. The cup (the part that touches the lips) should be free of dents and be completely covered with the plating. If any of the brassy coloured material is exposed because the plating has been damaged, DON'T USE THE MOUTHPIECE. The exposed brass can cause some skin grief. Note any numbers on the mouthpiece (7C, 10 1/2C, for example) and visit your music store to replace that mouthpiece. If there are no numbers, 7C is common for students. Useful accessories: a cleaning kit, mouthpiece pouch, valve oil (there's some new synthetic stuff now available and comes highly recommended), mutes (a big variety and they're all different).

Trombones: Brands, see trumpets, above.

Wipe down (see trumpets, above). Slides: the working part of the instrument; these are the long, tubular parts of the instrument. Carefully take the slide out of the case. Inspect it visually for dents, bends or bows in it. These need professional service. Look at the open ends of the slide. On one of the pipes, there will be a ring-like lock. Turning this lock will free the slide for playing. Pull the slide in and out about half way. It must move very, very smoothly. Visually inspect the inner slide, a chrome or nickel-plated part. This should be dry, clean. The chrome should not be broken or chipped. Look for excessive oil or grease on it; a pro should clean it off. Service to the slide is generally best left to a professional and keeping it working is critical. Mouthpiece: see trumpets, above. Useful accessories: see trumpets above. There are special lubricants for trombone slides.

Dents: see flutes, above.

Instruments not listed here have been intentionally left off. For other band instruments (French Horns, bassoons, percussion) talk a band professional like a teacher or community band or orchestra director. Check out the manufacturer's website, just Google the brand name.