I saw that ad and was pretty suspicious myself as I have a hard time believing Pinarello would use anything but Columbus tubing for their bikes. I assume there are no tell-tale signs that it is indeed a Pini----I am thinking of embossed details on the lugs or on the fork crown. If they are not there and the tubing is indeed 531, I would guess that it is a bike that has had some Pini decals put onto it.

No big deal really, I think that it looked good from the photos and the price was very good too...cannot wait for the photos.