Beginners: Beginning musicians may wish to have a beginners instrument, but after a few months, it should be apparent whether the student will continue with that instrument. My suggestion? RENT. Too often, parents will purchase an instrument for a child who only plays it for a week. Finding a place that will rent you a trumpet before you purchase is the best bet. Parents also tend to buy an instrument that is too advanced for a starting child. I've seen many parents become upset when the $3000 trumpet they bought is broken by their ten year old.

Intermediate: After a student studies for a while, and shows promise, it becomes necessary to buy an instrument. If a beginning student has a beginners instrument for too long, though, their playing may never pass the beginning stage. The important thing is knowing when to move up. For students who started in elementary school, the time to buy an Intermediate trumpet may be in middle school. By purchasing an instrument at a level higher than you play, you give yourself room for improvement and you give yourself the opportunity to grow with your instrument.

Professional: Most students, when they are ready for a professional instrument, can feel it. They know that their valves aren't as smooth as they'd like them, or that the instrument just doesn't perform as well as they'd like. Now it's time to buy the professional trumpet. Be prepared to spend more money and come to the music store prepared to know what you want and need.

http://www.river2u.com/musical_instruments/trumpets.htm?kid=407973776

Beginners: Many companies sell beginners instruments, and it would be safe to buy one of these online on E-Bay or elsewhere without really trying it out. The most common options are listed here:

Intermediate: A lot of companies don't recognize the intermediate level of playing, but this is a very important stage. For someone who started in elementary school, this should be purchased in High School. It is the lower-grade professional instruments.

Professional: The key to buying a professional instrument is trying it out first. Most music stores will let you try their trumpets out first, but be sure to bring your own mouthpiece. Play some scales and try a range of brands. The store should have someone on staff who can help you choose what's best for you, but it's important to know what sound and feel you're looking for.

There are many more brands such as ACCENT, B&S, JUPITER, AMATI and GIARDINELLI which are all good instruments. BEWARE OF FAKES! Make sure the person you are buying from knows about trumpets before you purchase

For more serious players, there are more serious questions, such as bore width, type of metal, and hundreds more minute details that can make a big difference to a discerning professional. I've put here a basic listing of what each characteristic does, so you can make your own decision as to what would be best for you.

Bore Type: There are three types of bores: Straight, Step or Multi bore, and Conical. The straight bore is the same width throughout. Most trumpets come this way. The step or multi bore can have several bore widths, making the blow tighter or more open depending. The Conical bore starts tight and opens to a wider bore width, making the blow tighter and creating a warmer sound.
Bore Size: A larger bore width requires a lot of breath, while a smaller width requires less breath, but can cause a loss in rich tones.
Flare: The flare of a trumpet varies by each brand. The only way to know what is best is to try it out.
Leadpipe: The leadpipe works simularly to the bore size. There are normal and reverse leadpipes. Reverse leadpipes are not actually reversed, but are longer to accomodate a different style of tuning slide. http://www.ibowtie.com/tmptleadpipes.html
Metal: Silver makes a trumpet sound brighter, while gold brings the tone down to a melow level. Brass is the normal construction for beginning instruments while silver is used most in intermediate or professional levels. Gold is only used in professional models unless used as an accent on silver trumpets. Then it's just there to look pretty and doesn't serve much of a purpose.
http://www.ehow.com/how_9558_buy-trumpet.html