If she doesnt have a dun parent, she's not a dun. If she doesnt have a parent with atleast one creme gene, she's not a buckskin.
However, on allbreed, her sire is listed as buckskin, and her dam a red dun... So if she did in fact recieve both the creme dilution and the dun modifier, she is what is called "dunskin" (Buckskin dun.). Registries do not yet recognize this, but it is the term for a black based dun with agouti and the creme gene.
If she does not have primitive markings (dorsal, leg barring, shoulder barring, or facial cobwebbing), she is not a dun... and she must have more than a dorsal to be considered a dun. If she does have the dun factor, but is more of a darker taffy color, she will more likely be simply a dun.
I do also believe your stud (if she was bred to him) is a cremello... so you wont be able to tell if she carries creme unless you hit the jackpot and got a cremello, perlino, or smokey black out of the mating.
Since you are selling her there is no point to test her... however that would be the only real way to test if she carries creme along with dun (provided she shows the dun factor to begin with).