WES website mentions that a transcript needs to be sealed and?
Most probably not. I had my transcripts evaluated by WES as well and based on the reviews I read before I sent my application out, WES is very strict when it comes to accepting or rejecting a transcript being received. The reason why they require a signature/seal across the backflap is to make sure that the transcript is legitimate and that the envelope has never been opened and resealed. I would not risk the application if you have unsigned envelopes. It can cause delays and additional hassle on your part.