I would take the new bike from the bike shop at a discount and keep it. Here's why:
1. Hopefully they can get a bike that fits you right, a good bike shop can do that and bike fit can make a big difference.
2. A good bike shop will include a 30 day tune up, so that after all your cables stretch a little from use (this is normal in a new bike) they can give it a quick adjustment.
3. If you don't know a lot about bikes, trying to flip a new one to trade down for a used one can have some pitfalls. You said you don't know the used bike market very well. Do you know how to tell whats up with a used bike? If you do your own mechanics, it's less of an issue, if you're paying a shop to do anything the used bike will need, it can add up fast, a full tune up can run $100 easy.
4. Personally, I'm not inclined to inviting that kind of hassle into my life for a potential $200 surplus. It sounds like work, and not fun work. Especially trying to trade a commodity that I'm not already super familiar with.
Just to complicate things, will they give you a deal on a Globe? It's a Specialized subbrand, they look like pretty nice commuter bikes.
Either way, Good luck!