Perhaps the salesman didn't know the Tacoma didn't have a real tow package, or maybe he was outright lying. We'll never know. For sure we know that the buyer should have done due diligence before he made the purchase. Its not like buying a camera.
One thing is for sure... that dealership is not going to take that trailer back without a whole lot of additional $$$ going their way. After you ask nice, you can demand, threaten, have your lawyer send them a nastygram, anything you want to do. It isn't going to happen.