yep.
As far as I know, yes, you can put a shopkeeper in any estate you own. Having a shopkeeper in apartment is actually what I did a while before I bought a booth... actually had 2 of em for a while. In theory, if you have the peds for it, you can do like Auktuma did for a while and fill an entire apartment with shopkeepers, making the apartment actually become the BIGGEST shop in all of entropia universe if you count item slots for selling thins... Good luck affording it, and finding enough of them to make it possible...
Shopkeepers actually work BEST in apartments since they eat 6 item slots each. In normal shops and booths, that's bad since you are giving up 6 to get back 20... with apartments, it's great since apartments have a lot of slots in them, but NONE of them can sell things until you put a shopkeeper there.
The other advantage of shopkeepers - they are mobile! I suspect they will work on apartments on other planets?