Hey,
As far as barrel trading goes, there are no legal issues to deal with.
When you trade the receiver, then you are trading a firearm. Then you have to comply with whatever local, state, and federal regulartions apply. Mostly, that means you have to use FFL dealers to ship out of state. In state transfers depend on your state. Note: Colorado recently put a law in place that says background checks must be conducted if you strike up a deal at a gun show. Other than that, it's basically cash and carry.
Where to find a barrel trade? Right here on this website! Just be careful about which forum you use, as the moderators prefer you do things in a specific manner. I'd recommend the "Barrel Trading" and "Classified" forums. You'll probably strike up a trade or sale very quickly. Be sure to specify caliber, barrel length, etc. when you advertise.
Trigger jobs? I don't have anything first hand to give you in that regard. Check back as several folks have a lot of first hand opinions to share with you in that regard.
B. Noz.