a lot of what you need to play trombone well needs to be demonstrated. You may very well learn to play to some degree, but if you want to develop a great sound and technique to where others will want to play with you, get lessons. The trombone is a relatively simple instrument, but it takes years to develop great intonation, slide technique, command of the range, and a great legato style. Don't sell it short.
i'd say that learning trombone isn't as simple as learning the make the noise and which finger to press when, you have to know exactly where everything is and what embrochure to use when. in other words if you want to get good i'd get a teacher
I honestly feel that anyone can teach themselves anything, as long as they have the dedication and drive to do it. I would highly suggest getting an instructor though. There is much more to the Trombone then just moving a slide that you might not pick up as well without someone to tell you the secrets of the trade =) good luck and best wishes =)