Your rep needs to be sterling or near to it to do those trades, and there are some techs that they want both arms and a leg for, or will not trade until the tech is long past its worth. Usually for me it is any of the governments, Nationalism, Espionage, Military Tradition, and Free Artistry.
Creepy, you just listed almost every single one of those which I research myself instead of buying.
For example: Because of my enemies being Scientific/popping huts nearby them that give them goodies and not me on higher levels, I usually waste 100 gold and around 2 gpt at just to get half-way through the Ancient Age by buying all the ones they found, and trading in-between nations tech for tech.
Then, I put all the commerce I have left over which is buy the time around 600 gold, and 40-50 gpt to get to Monarchy as fast as possible, get Polytheism, get Monarchy, and then trade Polytheism/Monarchy to construction and the other techs needed to get to Middle Age, from one person, and trade Poly/Monarchy to all the other people for money.
That way, I start out Middle Age as fast as everyone else, with around 100gpt, and 1000 gold?
Then I use THAT money to trade any Scientific empire that might've gotten Feudalism as a prequisite, and still have around 80 gpt.
At that turn, I'm having around 50 gpt self income, and 30 gpt income for the next 20 turns, which I use to in those 20 turns get around 3 techs all of which I monopolyse and sell for insane amounts of money, luxuries, right of passages, gpt, and all the other techs they might've researched at that point.
Then I sit back, at this point 100-150 gpt and not half-way through Middle Ages, let the treasure flow in, and give petty change in exchange for the techs needed to finish middle ages.
Start Indus. Ages, trade for Nationalism which some lucky fellar got, then go straight to Communism.
Rines and Repeat.
edit: In case you were wondering, I usually play continent games, and tend to make sure that the person on my continent has NO contact to the outside world, and annihilate him before Ancient Ages end while doing all this trading with the other continents, and I do THAT by rushing to Map Making, which I also forgot to mention.