Ambrotypes don't sell nearly as high as daguerreotypes. If it's a normal size (about 2x3 inch or 3x4 inch even) I think the offerer just likes it and wants it. I'd sell it for that and not waste time appraising it. You can do your own appraisal by seeing what similar items (animal shots on ambro or tintypes) have sold for. I bet it's less than $400. If it happens to be large, about 4x5 or larger, it's worth more. I'd sell now.