The magic altars from inner sea gods state that they lose their magic powers if moved.
So how do I set one up at a shrine or temple? Does it have to be craftet at the site where it will be set up or are they crafted inactive and start to work once they are in place?
They have a cost, so it seems possible to acquire one. But unless they are created "in situ" they have to be moved to reach the place where they are meant to be installed?
So if a cleric finds a sitz he deems suitable for a shrine or temple, can he buy an altar for his god, transport it to the designated site and activate it? Or does he need to bring the crafter to the place? If the latter, is that included in the cost?
I am pretty sure when you affix or move an altar you need to cast Consecrate on it (or Desecrate if its an evil deity's one). At least that is how altar get their power back in various adventures. Can't find any explicit RAW that says that ATM.
Treat it like any other magic item. The altar can be physically created anywhere but when it is enchanted it has to be in the location where it will be active. The enchantment process probably involves lots of rituals and appropriate sacrifices. For an evil god that may mean human sacrifices.
Despite having a price (for several mechanical purposes) these wouldn't be available on the market for the reason you are referencing.
Usually I would expect they are created by an important member of the religion in question that has the craft wondrous item feat, as such the cost is more used for determining creation cost than actually being something you would pay to buy.