Familiar is not an ability you can use in menus (that is why it is grey). This ability allows you to buy rare items from shops and it is used automatically, as soon as you get it all the shops` stock will be actualized to contain those rare items. Try visiting Timber`s pet shop, for example, and you will notice that some items have been added (mostly some J-scrolls) Regular shops only get a few GF items, the biggest benefit is from the shops in Esthar and perhaps Timber`r pet shop.
That is odd; perhaps you are confusing “Esthar shop” with “Esthar shop!!!” (if you are using the menu ability). They are 2 different shops with contain different stocks - one sells mostly ammo and the usual stuff and the other sells Elixirs and Cottages. The last one is usually closed and you must attempt to enter it several times before it finally opens.
EDIT: I think the one that sells Elixirs is called "Jonny`s Shop" in the shopping mall at Esthar