i would call the northeast document conservation center and ask them
who would appraise it for you. i have seen many ambrotypes over the years
and i think 800 seems a bit excessive, unless the person who took it is well known ...
but what do i know ...

they say that something is worth what someone will pay you for it so even if the
person who appraises it says it is worth 100$, you can easily put it on the auction site
with a hefty reserve and se