Ok first off the topic of using your home as a place of business is a little absurd for anything other than high demand cottage industry stuff (health potions and the like) so what I would do is divide the question.
A home is a place in game mostly used to keep your stuff.
You could easily implement this like AO (Anarchy Online) where they use an apartment system. You are given a key to a large building and in that building is really only one door. When you pass through the door, you are taken into your apartment. The apartment can be decorated, heck in AO you can even buy TV\'s and a Fridge and watch the big game if you want to.
In reality the Apartment can only be entered by the owner, however the owner can go to a machine that will duplicate his key, so he can give one to his friends (those who are paranoid can create a one time use key).
The data for the apartment can easily be stored client side, or could be implemented like a fancy backpack and stored server side. Eitherway the apartment is on it\'s own seperate map, so it could be expanded etc, without infringing on the original design. A great place for this would be in the upper levels of the Blood Tower

(Temple high rise anyone?)
As for a place to conduct business, that would be great. However you would want/need an NPC to man the store while you are away. With 100,000 players, the amount of Map space needed would be horrendous. (urban sprawl)
If I were designing an in game business though, I would do a simple consignment shop. Where there is a single guy in any town in game that takes any item you want to give him, and then you set a price. When someone comes to the store to buy and sees your item, the NPC takes the money, then notifies you about the sale. Then the game either places the money in your bank account (Direct Deposit?) or you come and collect the money from the NPC yourself.
The difference between this approach and the other ideas is that instead of each person setting up shop (which would actually waste alot of map space, not to mention being a programming nightmare) there is a single place to go to buy, and effectively be able to buy from All players at once. Also with a single point of contact an item is much more likely to be sold, especially if the player is offline.
Another slight variation would be a pawnshop, where if you are in a hurry to sell an item, the NPC could buy the item from you, for a percentage say 50% of what the same item sold for last time. This approach would guarantee that people would try to develop thier trade skills, since they would have a fair market in which to sell them, and an always interested buyer. But please make sure not to have NPC price gouging like in EverQuest where it costs 10plat (the currency in that game) to make an item that you can only sell it to NPCs for 100 gold (1plat = 100 gold) and then the NPC sells the same item for 20 plat. This system is very discouraging and one of the main reasons I quit playing EQ.
I hope this helps.