Buy'em all in 1 round:
After you finish moving or whatever you've done in game, DON'T CLICK BUTTON 'FINISH TURN', and then do trade with the second player and then cancel it.
You will find the button already change to 'Roll Dice'. So there you go!
If there are 3 cards of one colour and you have got one and the computer has got the other 2, the computer will trade with you. Don't click accept on seeing the hefty amount. To make it more hefty, click on the money thingy on the computer's part and increase the amount. This way you can earn up to 1000m.
Get tons of money:
First if you have one property and the computer has 2 others in same color, it will trade, decline it, then trade for it like 15 mil for green, then trade the full cost for it to get it back so it doesnt have a monopoly!