Allowing trades would really put a hamper on the transfer market.
Also, as Forrest said, being only able to trade within a certain salary range also defeats the purpose of trading in the 1st place, especially when a mechanism (the TL) already exists for transactions of this nature.
There's just too much foul play that can occur with trades for them to be worth the minimal benefit that is derived from allowing them.
                                
                                
                                
                                
                                
                                
                                    NO ONE at this table ordered a rum & Coke 
 Charles: Penn has some good people 
 A CT? Really? 
 Any two will do 
 Any three for me 
 Any four will score 
 Any five are live