Being able to pick up some resources, trade goods or strategic, from a minor or major, for a one time payment of credits or tech is boring.  I think such trades should be ongoing and subject to periodic review.  Also, it should obligate you to protect the minor you are getting it from if they get attacked.  For majors, you can monopolize the galactic supply of goods, but doing so should be either a constant drain on your economy (different trade goods, research drain, or credits/turn) or subject to recurring costs (such as credits, technology, or ships).  It should also require some level of treaty arrangement (influence tech), and the resource trade route from their planet to your capital should be subject to plundering and blockades.