Don't ever buy an instrument on eBay unless it's from a retailer you already trust. Too many things can be wrong that you won't find out about until it's too late.
There have to be some rental places in your area. When I was a kid, I started with the trumpet. My mom rented me one, and turns out I didn't like it. I moved over to saxophone, and after I stuck with it for two years, she bought me one. I still play it, fifteen years later.
Rent first. That's the way to go. My band teacher was able to recommend a music shop - perhaps your son's can as well?
And congrats. Music will open up a whole new world for him. It was a great gift that my mother gave me.
What didn't you like about the trumpet? I want to learn a new instrument (I play piano and violin), and have been going back and forth between the trumpet and the banjo.