First of all: I'm very sad to hear the passing of Steffi... may she rest in peace.
Regarding the pony: I think you did the best by choosing to auction her off. Just as allymer said, signatures as collectibles can be so dividing: some people will look at items with a signature on as something damaged, even if the signature is not personalised, and even if they like and respect the person who signed the item, while others will be willing to pay much more for a signed item - but how much more? It depends on whether you find the right buyer who is looking exactly for this, and how much such an item is worth for them. I hope Blossom will go to a good home where she will be respected for her extra feature