I recently bought a GAD50PCE with the "Used" stamp on it as well. I had a '95 D4 and a '96 F4 before this. I loved the D4, but wanted a guitar with a cutaway. I liked the F4 for its great playability but missed the dreadnought sound, so I got this GAD. I also am amazed at the the quality, tone and feel of this instrument. I have friends with Taylors (414CE, 614CE) and Martins (D-28) and I'd take this one over any of them. This surprises me with the 614 since it's a much higher end guitar than the other 2 which are about the same list price as my GAD.
I've heard that the "Used" or "2nd" stamp on them means that there was either a flaw found in the guitar at the factory or it was shipped to a distributor who didn't sell it and it went back to the factory where it was stamped. I've looked for a flaw in mine and can't find anything. It's a beautiful guitar, so I have to conclude I got one that didn't sell. Which is fine with me since I paid about 1/3 list. If anyone knows for sure what the "Used" stamp means, I'd appreciate the information.
Mike