In my opinion this will just cause even more questions and confusion among the players, not to mention people asking ''why can't i use the trade chat'' in adition on ''why can't i trade?'' If that for example is implemented, it wouldn't affect only the trade channel but the whole chat overall. People need a picture how much something costs and some do like to get some pokes ready in advance or simply do private price checks. I personaly didn't encounter that many trolls/spam accounts that it would require a restriction for a badge based permission to use the channel. Maybe the idea has some potential but I don't see it being urgent.
Thats easily solved by a pop up message that says'You need 4 badges to trade or use the trade chat' or something along this line whenever someone with less than 4 badges attempts to post on the trade chat.
Its not like they are prevented from checking the trade chat or pm other players to do price checks so they can do what you said,this suggestion only restricts them from '''posting''' in the trade chat when they cannot trade to begin with,so they do not waste others players time as well.
Then you got lucky and logged in when they are not logged in,also the number of troll accounts is not important,as having this requirment solves 2 things.
1)People posting trades when they cannot trade(whatever they know they cannot trade or not)that solves the problem of wasting both people's time.Not to mention whats the point of being able to post on the trade chat when you don't have 4 badges?Its not like you/they miss on anything other than wasting people's time.
2)No matter how big or small in number those troll accounts are,at the least with this requirment the trade chat will be safe from them,as they would not bother to get 4 badges only to get banned.