A couple of comments...
It has been mentioned that it is unrealistic for the trade income for a planet to be larger than its gross production without trade. I don't actually have a problem with this on the grounds that that planet might have beaches covered in diamonds instead of sand. Or streets paved with dilithium. Or any other item you might think of.
In fact, the sand on our beaches might be ridiculously priceless to them! Who Knows.
Bu seriously, on the subject of 'gamey' tactics. It is really difficult to split the hairs between gamey and non gamey. There are, however some guidelines. Here are some ideas.
INSECT LOVERS FORBIDDEN
First, it is inappropriate to take advantage of a bug in the game. Of course, some people disagree about what is a bug and what is a feature, so it is best to list what bugs can be exploited and have a gentleman's agreement not to exploit them.
NO ABUSING THE ABBOS
Second, it is inappropriate to take advantage of the AI. This one is even harder to define. It is my opinion, that it is best to not even have AI in a multiplayer game. However, sometimes it is advocated by some of the players and must be dealt with. If you do, you need to be sure and turn off surrender. There are too many ways to pump up your score and make them surrender. Also, we play with an agreement that you may not offer a trade to the AI other than technology.
I have seen some rather spectacular AI exploits involving claiming a system you can't control and trading the rights to that systme for an AI homeworld. In defense of the person who did this, their opinion was 'everyone can do this, I might as well also'. I believe that person honestly felt that way and I am not sure I can argue with it. This is at the root of having these discussions. We want to define what is right and wrong so we will do everything everyone else will do.
THERE CAN ONLY BE ONE
I have tried to have an opinion about the 'gaminess' of something by asking myself 'Would an empire do this if this were reality?' Again, this quickly gets into a real gray area. One simple way of accomplishing this is to play with the rule that there can only be one winner. This means that there are no shared victories and no points for second place.
I'LL SHOW YOU
I have played games (this and others) in which someone always felt it was their duty to 'even the field' as they were drummed out of the game. If you are about to be conquered by a player and you try to give what is left of your empire to someone else, you have to ask yourself if this is realistic. Would an empire gift its possessions to someone else when there was any hope? If there is value to the gift, is there truly no hope?
Hope I didn't bore you.
Have a great day!