You can already buy limited availability houses and sell them later for a markup when they are in high demand. You can own as many houses as you want I think..
I'm not sure if anyone has done it before but as more people get to the endgame we are more likely to see it. I think this might be one of the reasons why we have limited availability for the houses in the first place though.. we are supposed to use the house market like this^^
I think if a feature like this were to be implement the houses would need to restock more often though, because there would be a higher demand^^

For your other point, that's what I meant with "not the pixpet/pumpkin/plant slots" Obviously you should not be able to grow pumpkins in all of the houses, just one. I would like to be able to place pixpets in the other houses for deco... but we should not be able to send them on a hoard and they should not count towards badges^^