I discovered this tactic (or exploit?) to be very powerful in a tiny map, where civs usually have at most two planets each, and you have the upper hand in diplomacy and tech.

1) Arrange for a surprise invasion on your chosen opponent, by having super long range troop transports and attackers to dispense of those pesky orbital defenders. Have enough ships and enough speed to be able to take ALL of the civ's planets within one turn. It is crucial that the speed is high enough so that the ships can be positioned far far away and the civ won't suspect this invasion coming.

2) at the start of that turn, trade most of your techs & trade goods for their ships, techs, money, and influence. If you are lucky, you might even manage to buy off their planetary defenders! Heck, I even got to buy their troop transport in orbit, effectively giving me 1000 of their own citizens to invade them right afterwards... oh the irony of it all! Note that if you have your troop transports too near, they will refuse to trade their ships.

Note: DO NOT trade planetary invasion techs, as it will decrease the odds of you capturing the planets. Also, save the diplomacy techs for last, coz i believe if you trade em first, they will learn how to bargain harder and thus you get less money out of other techs. Also, more tediously, you gain more cash if you trade your own item one by one rather than in bulk. Start with bulk first, since that might net you their ships, starbases, etc. Also, trade progressive techs in stages. they wont accept Laser 1 and Laser 2 together, for example.

Essentially, you are salvaging the resources and wealth of a civilization that is about to be wiped out anyways, so why let their stuff goes to waste? If you do this right, you will end up owning most of their cash, substantial influence, their techs to complete your collection, and if you're lucky, their ships and starbases as well to help in slashing their own throat. They themselves will end up being 2nd most technologically powerful civ in the galaxy, also the one with the shortest timespan holding that title

Of course, if they survive this invasion, then you are up for an interesting aftermath session...

Repeat this with every civ, especially after everybody has built their own capital buildings and super projects, allowing you to own more than one of these neat stuffs all over your empire. Imagine having 3 hyperion shrinkers, not to mention having manu and tech capital on every single planet you own...