Darn, well, if they don't have cash, why not ask them to give something else, like starships or technology(or maybe even... planets)? 3 economic starbases ought to be worth at least SOME ships/tech. I plan to build 4 ESBs per trade-route sector, selling 3 and keeping 1, except for the sector containing my planet, in which case I'll mass the SBs at my planet and add some production modules as well.
i thought you were going to play with tech trading off

they don't give up techs easily. by the time you had enough SBs to be worth a tech, you'd either likely have aquired it elsewhere else (and need more SBs to equal a better tech), or you'd have suffered from not having it all that time.
i really think the only way to make a trade array useful in a real game is to build it on a very short set of routes and saturate them as much as possible. otherwise it'll simply take too long to develop. having built from scratch arrays that span an entire gigantic map, i can say from expereince that it's only possible to do once you've already pretty much secured your victory. by that time, even the numbers i was producing wouldn't be a major addition to the economy - though it is a nice perk that trade doesn't seem to be cut by 20% when your treasure exceeds 20K.
i personally feel AI ships are worthless, though i don't mind selling my out-dated ships to the AI (often a lot easier than waiting around to upgrade them). the only ones i ever buy are their default mining ships, and usually that's only because i flip a planet while they're creating a asteroid base, and buying the miner is the only way the game realizes the miner needs to be moved (a real pain the rear).
favors wouldn't be a bad thing. "hey, freaky alien, i'll give you a bunch of starbases if you attack this other jerk i don't particularly like." it's also a good idea to keep the AIs at each other's throats, especially in a diplomatically oriented game.
as for planets, that's indeed a viable option - under the right circumstances. the AI seems loathe to trade away its well developed and high-quality planets. as a neutral, what you'll want to do is conive them out of their low-quality, unterraformed planets (PQ4 and under). they'll give them up as worthless, most likley, because even when they've researched up to terraforming they just let the terraforming tiles accululate and don't accelorate the development of their lower-quality planets. just be careful not to grab planets that will merely flip back.
however, if you do find a useful way to implement some of your ideas, please do share! i'm always interested in trying new things and hearing about things i've never tried myself.
cheers