The most listed ratio I've found on the forums is 3:2:1
The merchant is 4:2:1

What is the leeway given in player to player trades? Must we strickly follow the ratio, land in between the two, or do we have some percentage that we can adjust in making the deal.

According to the link from sloth, 4:2:1 is acceptable (as 3:1,5:0,75 - all are included in allowed range)