I don't think this is necessarily suspicious. But...
1) Based on the length given for the seat tube, it would be somewhere between at 53cm and 54cm frame. Trek made (makes) a 54cm frame, but visually it looks more like either a 56cm or 58cm frame. Ask if they will measure from the center of the crank to the top of the seattube where the collar would go.
2) The price might end up being good, but do you really want to buy a carbon frame from a place that can't even give you the history? Remember, frame warranties typically only belong to the original purchaser, and they normally don't apply if the frame was abused.