Pretty much what they said is correct.
You can have a "business" without any sort of actual storefront, house or guild hall. Most crafters are in some way salespeople as well.
People have run businesses out of player housing, which has been mentioned is in extremely limited supply, especially on West. I think the cheapest house available is something like 45,000 or 50,000 GPs (I could actually look, but I'm too lazy right now). Housing on central starts at something like 100,000 or more. There is no player housing on East whatsoever.
You can start a guild, as has been mentioned, but you must abide by the rules and qualifications of guilds in the game. With enough money, your guild can establish a guild hall, which will generally cost somewhere between 500,000GP and 1,000,000GP. Only one guild has established a dedicated storefront in-game, and that was the Orc Bashers, and that was only after something like a RL year of being an active and stable guild, and I'm not sure how much it cost.
I hope that helps.