There are no exploits in trading.
There are simply good and bad traders.

If you found a way to make trading more profitable, then you just became a better trader.

some techs you only pay or get paid for the top tech of that tree you offer/request. weapon techs are all this way.

for instance, if you try and sell graviton driver 3 and gd2 and gd they only pay for gd3. on the flip side if you buy GD3 instead you can get every lower mass driver tech for free.

this is just one of the reasons many ppl turn off tech trading, the ai can take a real financial beating by using this fact. the other thing is that sometimes the higher tech if cheaper than the lower one. in your example harpoon costs 1200 to research but harpoon2 only costs 800. since you got harpoon for free when you bought harpoon 2 you ended up paying less for it

obviously this is meant to represent the dificulty of pioneering a tech rather than just refining a developed one, reasonable. but it does result in the situation you stumbled on. their solution isn't perfect, so sue them or turn off tech trading if it really bothers you

if you try and sell graviton driver 3 and gd2 and gd they only pay for gd3

You don't sell 2 of the same tech in the same trade round.

Sell the expensive one first, then in the next round sell the other, you get full "price" for both.