I play on normal right now and I sell every tech I get. I research some of the main ones to give me new ships but I also research all the smaller ones too. I then sell them to any AI with at least 100bc in it's treasury. I try to get 100 bc for as many turns as I can, even if it is only 4 or 5 (like it when I can get 20 or 30

).
I trade with everyone, even if it helps boost their combat abilities (except gravity accelerators and aphrodisiacs). I frequently swap several techs for transports and ships with enemies I'm almost ready to take on. They'll sell almost any of their ships if you have techs they value highly (ship designs, weapon upgrades, shields, etc.) I usually have 800 to 900 bc coming in every turn. If it drops below 600 I go whoring again.
I especially like the minor races because they have major cash. This also drives their economies into the ground so they hardly build wonders or trade goods before I can. This also allows me to run my economy at 100% for almost the entire game. I leave tech spending at 40-50% all the time and trade for any tech, even if I don't get a good deal if it opens new research areas.
BTW, buying transports and combat transports with full loads can be extremely effective because you can get 4 or 5 from each civ without impacting your population. They also are typically within striking distance of empires I'm ready to take out. I typically never get war declared on me because I sell to everyone equally, so I can mass my forces and pick off an empire at a time.
You'll be surprised how quickly ships and improvements are built around the middle of the game if your spending is at 100%. I also usually end up with around 150bc extra each turn with my treasury getting as high as 15000bc. (Really handy for those quick purchases.) This also lets me colonize 13 and 14 worlds because I buy soil improvement and habitat imp immediately. My social spending usually stays around 20%, and this builds my core planets up to the max.
~SDC~