When you get a good deal on a slightly used bow, you're getting it after the initial depreciation has happened. I think it's a good way to go. If it's a current model and you find it at a proshop that can fit it to you, what do you have to loose? I took my time and watched the used rack and ended up with a barely used Martin with some decent accessories from a Martin dealer and he agreed to honor the warranty. I got a really sweet bow at the price of a much lesser model. Martins are affordable to begin with but I feel like I really got my moneys worth.
Then there are these guys who have found ways to avoid that depreciation factor all together. But if you want something to learn with you aren't really in their shoes as a consumer.
Remember that you'll need a lot more than just a bow.
You'll get lots of good advice coming here for sure.