I am not aware of any prohibition from referencing the ChipGuide number (CG:) in a auction.

However, I would use a little bit of caution because a "CG #" can change, unexpectedly. There many older entries in ChipGuide with inferior scans. If a new scan is submitted and looks to be a different example than the existing scan, then the new scan gets added as a new CG #. Later, the duplicate entries are found and one get deleted, along with the associated CG #.

I mention this, just so that you are aware. However, using a CG #, as a reference, should not be a problem. And although you have NOT suggested this, just in case anyone is confused by my comment, it is the reference number that is OK to use, NOT the ChipGuide scan.

Jim