I have bought and sold ecto from and to other players fairly regularly for years. I buy it when getting low on room to hold more gold in storage and sell it when getting ... well, too much room to hold gold in storage.

I have also used ecto as payment for trades, both as a buyer and as a seller. I have done this as recently as yesterday.
I have never once had the slightest hint of trouble.
I don't work for ANet, so I can't say to you there is absolutely no chance this will be a problem. However, as a long time player who's never had a problem I can say the odds of you getting in trouble for receiving ecto in a legitimate trade which was acquired illegitimately in the past seem very low. Also, it makes sense. Players trade in ectos all the time. We have no way to see any kind of past trade history for the ecto in the trade window. It would be incredibly unreasonable for ANet to hold you responsible for what someone else did.
/offtopic
Darcy, you should ban End now ... not for naming sites or anything ... just an lolban to keep him on his toes, ya know?