Because condos are a bragging right. There wouldn't be a way to tell the difference between a piano that someone worked hours to purchase, and the other one where a guy just bought it and plopped it in. If you buy everything for your condo without working for it, half of the game is gone; buying shit is the incentive for playing game worlds. Players like to admire other peoples condos, that's the entire mentality of the genuine meter.
Selling ordinary items is basically the same thing as having a way to purchase GMC directly with real money. The market would have to be limited if we wanted to implement it. Regular furniture would be sold through Tower, not the market.