One Investigator (who has not had her turn yet in the Movement Phase) is in a street area with a Monster. Another Investigator is passing through that same street & has successfully evaded the Monster. Are the 2 Investigators now allowed to trade with each other? Could the trade have taken place prior to checking for evasion? I would say "yes" to both but I want to make sure because I was wrong about the Hound of Tindalos.

You are allowed to trade. You are even allowed to trade before attempting the Evade check. The conditions to be met are:

a) that you're in the Movement phase

b) that you are in the same area as another investigator

As long as these two are met, you may trade