How do I go about buying a used trumpet?

I’d like to get my girlfriend a really good trumpet for her birthday. Unfortunately, a quick look tells me that they’re crazy expensive, and out of my price range. I’d like to try buying a good one used on craigslist, but I know nothing about what to look for. Can anyone help me with the details of what’s important when it comes to trumpet buying?

I’d like to spend less than $500. She also has a trumpet already—I think it’s just not that good. Passable.