It will show the amount you put up, and then that amount will decrease as it's bought. Of course, AI/players need to click the "buy" buttons to actually buy from it

It should be in your gameconstants file, I'm at work so I don't remember which folder it's in. Someone else can probably find it for you
So, what if the market price is very high. Does that mean I need to set my price to 150%? I've noticed I can put it up to 150% and it still says I'm the lowest available resource.
As for tradeports, I wish you could select ALL the trade ports you own, all 50 or so I had, and then switch them to resource boost. That would have given me an absolutely insane jump in crystal/metal, but reduced my income to almost nothing or negative. As it stands I think you can only select the ports within 1 planet.
I didn't know you could do multiple trade ports until last night. Once I found that out I realized just how much you could do with economy in this game. I seriously put 4-6 trade ports in all non-front combat cities and my income went through the roof. It was so addicting seeing my trade output 2x or more that of my planet output.
Plus, as advent, desert planets are now more useful then terran planets. They can hold 36 logistic slots where terran planets only hold 24. Thats 9 trade ports on one planet. Take into account that each trade port brings in I believe 1.6 credits/sec, that's 14.4 credit/sec. The desert planet also gets 7.5 when fully populated with all tech upgrades. So, desert = 21.9 credit/s, terran = (10 + 9.6) = 19.6. I guess its not huge, but its nice that a desert planet is so useful for advent.
Not to mention advent desert planets as front line planets rock. You can put up 5 frigate yards and a capitol yard and still have room to spare with culture and be able to produce huge fleets in now time. I was using the advent's best tactical weapon, the "culture bomb, laser of love, w/e" and shooting it at a distant homeworld. After I hit it about 8 times, the planet's allegiance dropped to 0% and blew up as well as the rest of his planets. I didn't know it would have that effect, nothing like blazing some hippy culture bombs and blowing their minds.