HI!
IMHO there are only 4 reasons sellers do not ship to APO addresses:
1. They don't know what it is, and they just say no, instead of asking what it is.
2. They don't ship USPS.
3. They are scared away because DC is not available. They fear the PAYPAL chargeback.
4. They think it costs more and is complicated.
I for one ship to APO without a problem, and this is listed in my shipping terms.
If I was you, I would draft a short simple email asking sellers if they ship to APO's, and explain what an APO address is in the email. Especially that the postal cost is the same as shipping to a regular address with that zip code.
Some people simple don't know that this is how servicemen/women receive their mail. That it does not cost any more to ship to them. The package is fully insurable.
They only things you do different is to fill out a customs form, and you hand write the label.
I understand in the near future that DC is going to be available, and this will free up many sellers to ship to APO's.
Good Luck!
Beadmistress