yer unfortunately too many people would take advantage of new unssuspecting users.
Ill trade you awesome player X for your awesome player Y!
When in actual fact, awesome player X is awesomely shizen and not worth a quarter of awesome player Y.
And it also opens up a lot of options for cheating etc as other person has said.
There are plenty of discussion in the suggestions forum, i would go in there and do a search for player trading as a topic and have a read of why it has never been implemented.