I have bought several Damascus barreled guns from dealers. None had me sign anything other than the BATF form 4473 (these were not pre 1899). None related to me any warning about What to or Not to shoot out of them. Beyond that someone else will have to answer.
I am not personally aware of a dealer refusing to sell a damascus barreled gun but perhaps if there were none on the racks, this was why & I just didn't know the reason.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra