Usually when I join forums I use my email address, 'esnible', but I was afraid to with Tantalus. This is because my friends google me. Although they know I collect coins it seems foolhardy to display my purchases and values online. It also works the other way -- if someone sees a coin on Tantalus it is pretty easy to find my real name and address, and thus the current location of the coin.
For example, the first coin I see in Tantalus is http://www.tantaluscoins.com/coins/17625.php
. The 'entered by', 'added on' are the fields I'd want to keep private. The suggestion of 'semi-private' is to hide those fields.
In my case there is only one realname for esnible (easily findable by googling) and only one person with that name (address easily findable via zabasearch.com). For folks with more common names this is less of a problem. Zabasearch finds a Rasiel Suarez, about your age, but not in Asheville.
The other privacy concern goes in the other direction. My wife knows about my coin purchases but not all collectors work that way. Some collectors would rather keep their investment habits more private, but don't mind if folks use the pictures for attribution.