The key to a good trade-driven economy (for any faction, not just TEC) is to build a long trade route. Early on, you should only build trade ports which expand your trade route. This gives you the most bang for you buck, and if you set up your logistics right allows you to get by without purchasing any expensive logistics ugprades on your planets.
Be careful when planning your trade route, since adding a new trade port may accidentally make your trade route shorter. Traders always take the shortest route from point A to point B, so building a trade port that opens up a short-cut could reduce the length of your trade port chain. Take for example this hypothetical system:
A = B = C = D
|| || ||
E = F = G = H
Suppose our trade route is currently B-A-E-F-G-H-D. Because C does not have a trade port there, it is as if it doesn't exist at all with respect to trade routes. This gives us a wonderful trade route of 7 jumps. However, if we place a trade port on C this is no longer viable. The shortest path between B and D is now a mere two jumps, and the path between them is no longer our longest trade route. Instead, we get the (much shorter) A-B-C-D-H route which is only 4 jumps.
I know this is somewhat confusing, trade routes always take a while to figure out.